rustc: Add official support for weak failure

This commit is part of the ongoing libstd facade efforts (cc #13851). The
compiler now recognizes some language items as "extern { fn foo(...); }" and
will automatically perform the following actions:

1. The foreign function has a pre-defined name.
2. The crate and downstream crates can only be built as rlibs until a crate
   defines the lang item itself.
3. The actual lang item has a pre-defined name.

This is essentially nicer compiler support for the hokey
core-depends-on-std-failure scheme today, but it is implemented the same way.
The details are a little more hidden under the covers.

In addition to failure, this commit promotes the eh_personality and
rust_stack_exhausted functions to official lang items. The compiler can generate
calls to these functions, causing linkage errors if they are left undefined. The
checking for these items is not as precise as it could be. Crates compiling with
`-Z no-landing-pads` will not need the eh_personality lang item, and crates
compiling with no split stacks won't need the stack exhausted lang item. For
ease, however, these items are checked for presence in all final outputs of the
compiler.

It is quite easy to define dummy versions of the functions necessary:

    #[lang = "stack_exhausted"]
    extern fn stack_exhausted() { /* ... */ }

    #[lang = "eh_personality"]
    extern fn eh_personality() { /* ... */ }

cc #11922, rust_stack_exhausted is now a lang item
cc #13851, libcollections is blocked on eh_personality becoming weak
Make configure respect (and save) values for `CC`, `CXX`, `CFLAGS`, etc.

I mostly tried to remain backwards compatible with old invocations of
the `configure` script; if you do not want to use `CC` et al., you
should not have to; you can keep using `--enable-clang` and/or
`--enable-ccache`.

The overall intention is to capture the following precedences for
guessing the C compiler:

 1. Value of `CC` at make invocation time.
 2. Value of `CC` at configure invocation time.
 3. Compiler inferred at configure invocation time (`gcc` or `clang`).

The strategy is to check (at `configure` time) if each of the
environment variables is set, and if so, save its value in a
corresponding `CFG_` variable (e.g. `CFG_CC`).

Then, in the makefiles, if `CC` is not set but `CFG_CC` is, then we
use the `CFG_CC` setting as `CC`.

Also, I fold the potential user-provided `CFLAGS` and `CXXFLAGS`
values into all of the per-platform `CFLAGS` and `CXXFLAGS` settings.
(This was opposed to adding `$(CFLAGS)` in an ad-hoc manner to various
parts of the mk files.)

Fix #13805.

----

Note that if you try to set the compiler to clang via the `CC` and
`CXX` environment variables, you will probably need to also set
`CXXFLAGS` to `--enable-libcpp` so that LLVM will be configured
properly.

----

Introduce CFG_USING_CLANG, which is distinguished from
CFG_ENABLE_CLANG because the former represents "we think we're using
clang, choose appropriate warning-control options" while the latter
represents "we asked configure (or the host required) that we attempt
to use clang, so check that we have an appropriate version of clang."

The main reason I added this is that I wanted to allow the user to
choose clang via setting the `CC` environment variable, but I did not
want that method of selection to get confused with the user passing
the `--enable-clang` option.

----

A digression: The `configure` script does not infer the compiler
setting if `CC` is set; but if `--enable-clang` was passed, then it
*does* still attempt to validate that the clang version is compatible.

Supporting this required revising `CLANG_VERSION` check to be robust
in face of user-provided `CC` value.

In particular, on Travis, the `CC` is set to `gcc` and so the natural
thing to do is to attempt to use `gcc` as the compiler, but Travis is
also passing `--enable-clang` to configure.  So, what is the right
answer in the face of these contradictory requests?

One approach would be to have `--enable-clang` supersede the setting
for `CC` (and instead just call whatever we inferred for `CFG_CLANG`).
That sounds maximally inflexible to me (pnkfelix): a developer
requesting a `CC` value probably wants it respected, and should be
able to set it to something else; it is harder for that developer to
hack our configure script to change its inferred path to clang.

A second approach would be to blindly use the `CC` value but keep
going through the clang version check when `--enable-clang` is turned
on.  But on Travis (a Linux host), the `gcc` invocation won't print a
clang version, so we would not get past the CLANG_VERSION check in
that context.

A third approach would be to never run the CLANG_VERSION check if `CC`
is explicitly set.  That is not a terrible idea; but if the user uses
`CC` to pass in a path to some other version of clang that they want
to test, probably should still send that through the `CLANG_VERSION`
check.

So in the end I (pnkfelix) took a fourth approach: do the
CLANG_VERSION check if `CC` is unset *or* if `CC` is set to a string
ending with `clang`.  This way setting `CC` to things like
`path/to/clang` or `ccache clang` will still go through the
CLANG_VERSION check, while setting `CC` to `gcc` or some unknown
compiler will skip the CLANG_VERSION check (regardless of whether the
user passed --enable-clang to `configure`).

----

Drive-by fixes:

* The call that sets `CFG_CLANG_VERSION` was quoting `"$CFG_CC"` in
  its invocation, but that does not play nicely with someone who sets
  `$CFG_CC` to e.g. `ccache clang`, since you do not want to intepret
  that whole string as a command.

  (On the other hand, a path with spaces might need the quoted
  invocation.  Not sure which one of these corner use-cases is more
  important to support.)

* Fix chk_cc error message to point user at `gcc` not `cc`.
Add non-utf8 byte to the bytes!() example

Only an example was needed, as the ability to write uints into the string is
already mentioned.

Fix #7102.
auto merge of #14305 : tbu-/rust/pr_doc_bytes, r=huonw

Only an example was needed, as the ability to write uints into the string is
already mentioned.

Fix #7102.
auto merge of #13823 : pnkfelix/rust/fsk-fix-13805, r=alexcrichton

Make configure script respect (and save) values for `CC`, `CXX`, `CFLAGS` etc.

I mostly tried to remain backwards compatible with old invocations of
the `configure` script; if you do not want to use `CC` et al., you
should not have to; you can keep using `--enable-clang` and/or
`--enable-ccache`.

The overall intention is to capture the following precedences for
guessing the C compiler:

 1. Value of `CC` at `make` invocation time.
 2. Value of `CC` at `configure` invocation time.
 3. Compiler inferred at configure invocation time (`gcc` or `clang`).

The strategy is to check (at `configure` time) if each of the
environment variables is set, and if so, save its value in a
corresponding `CFG_` variable (e.g. `CFG_CC`).

The `configure` script also does not attempt to infer the compiler if
`CC` is set; but if `--enable-clang` was passed, then it *does* still
attempt to validate that the clang version is compatible.

Then, in the makefiles, if `CC` is not set but `CFG_CC` is, then we
use the `CFG_CC` setting as `CC`.

Fix #13805.
Add .isatty() method to StdReader

StdWriter has .isatty(). StdReader can trivially vend the same function,
and someone asked today on IRC how to call isatty() on stdin.
Remove useless ToPrimitive bound on range_inclusive()
Remove Rng.choose(), rename Rng.choose_option() to .choose()

Rng.choose() is used so rarely that it doesn't necessitate having two
methods, especially since the failing, non-option variant also requires
Clone.

[breaking-change]
auto merge of #14293 : alexcrichton/rust/weak-lang-items, r=brson

This commit is part of the ongoing libstd facade efforts (cc #13851). The
compiler now recognizes some language items as "extern { fn foo(...); }" and
will automatically perform the following actions:

1. The foreign function has a pre-defined name.
2. The crate and downstream crates can only be built as rlibs until a crate
   defines the lang item itself.
3. The actual lang item has a pre-defined name.

This is essentially nicer compiler support for the hokey
core-depends-on-std-failure scheme today, but it is implemented the same way.
The details are a little more hidden under the covers.

In addition to failure, this commit promotes the eh_personality and
rust_stack_exhausted functions to official lang items. The compiler can generate
calls to these functions, causing linkage errors if they are left undefined. The
checking for these items is not as precise as it could be. Crates compiling with
`-Z no-landing-pads` will not need the eh_personality lang item, and crates
compiling with no split stacks won't need the stack exhausted lang item. For
ease, however, these items are checked for presence in all final outputs of the
compiler.

It is quite easy to define dummy versions of the functions necessary:

    #[lang = "stack_exhausted"]
    extern fn stack_exhausted() { /* ... */ }

    #[lang = "eh_personality"]
    extern fn eh_personality() { /* ... */ }

cc #11922, rust_stack_exhausted is now a lang item
cc #13851, libcollections is blocked on eh_personality becoming weak
core: Stabilize the mem module

Excluding the functions inherited from the cast module last week (with marked
stability levels), these functions received the following treatment.

* size_of - this method has become #[stable]
* nonzero_size_of/nonzero_size_of_val - these methods have been removed
* min_align_of - this method is now #[stable]
* pref_align_of - this method has been renamed without the
  `pref_` prefix, and it is the "default alignment" now. This decision is in line
  with what clang does (see url linked in comment on function). This function
  is now #[stable].
* init - renamed to zeroed and marked #[stable]
* uninit - marked #[stable]
* move_val_init - renamed to overwrite and marked #[stable]
* {from,to}_{be,le}{16,32,64} - all functions marked #[stable]
* swap/replace/drop - marked #[stable]
* size_of_val/min_align_of_val/align_of_val - these functions are marked
  #[unstable], but will continue to exist in some form. Concerns have been
  raised about their `_val` prefix.

[breaking-change]
auto merge of #14259 : alexcrichton/rust/core-mem, r=brson

Excluding the functions inherited from the cast module last week (with marked
stability levels), these functions received the following treatment.

* size_of - this method has become #[stable]
* nonzero_size_of/nonzero_size_of_val - these methods have been removed
* min_align_of - this method is now #[stable]
* pref_align_of - this method has been renamed without the
  `pref_` prefix, and it is the "default alignment" now. This decision is in line
  with what clang does (see url linked in comment on function). This function
  is now #[stable].
* init - renamed to zeroed and marked #[stable]
* uninit - marked #[stable]
* move_val_init - renamed to overwrite and marked #[stable]
* {from,to}_{be,le}{16,32,64} - all functions marked #[stable]
* swap/replace/drop - marked #[stable]
* size_of_val/min_align_of_val/align_of_val - these functions are marked
  #[unstable], but will continue to exist in some form. Concerns have been
  raised about their `_val` prefix.
auto merge of #14315 : kballard/rust/stdreader_isatty, r=alexcrichton

StdWriter has .isatty(). StdReader can trivially vend the same function,
and someone asked today on IRC how to call isatty() on stdin.
auto merge of #14316 : kballard/rust/range_inclusive_no_toprimitive, r=alexcrichton
Updated doc with correct type.
auto merge of #14319 : kballard/rust/rename_rng_choose_option, r=alexcrichton

Rng.choose() is used so rarely that it doesn't necessitate having two
methods, especially since the failing, non-option variant also requires
Clone.

[breaking-change]
rustc: rename the lint `level` enum for style.

CamelCase all the way!
rustc: ng -> gn, fix a typo in a string.
rustc: improve error messages from wrong --pretty flowgraph use.

This defers to .fatal and .span_fatal for errors (rather than `fail!`
which prints the ICE message). It also adds the span lookup when an id
doesn't correspond to a block, to show what it is pointing at.

It also makes the argument parser slightly looser, so that passing
`--pretty flowgraph` recognises the `flowgraph` part and suggests to use
an integer.
auto merge of #14324 : zecozephyr/rust/docfix, r=luqmana
auto merge of #14326 : huonw/rust/tiny-fixes, r=pnkfelix

The changes to flowgraph make invalid invocations slightly more forgiving by (trying to) provide slightly more information and by avoiding the ICE message.