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
std: Rebuild sync::deque on Arc

This also removes the `&mut self` requirement, using the correct `&self`
requirement for concurrent types.
std: Rebuild mpmc queues on Unsafe/Arc

This removes usage of UnsafeArc and uses proper self mutability for concurrent
types.
std: Rebuild mpsc queue with Unsafe/&self

This removes the incorrect `&mut self` taken because it can alias among many
threads.
std: Rebuild spsc with Unsafe/&self

This removes the incorrect usage of `&mut self` in a concurrent setting.
std: Use Arc instead of UnsafeArc in BlockedTask
std: Build Exclusive on Arc<Unsafe<T>>

This removes the usage of UnsafeArc
std: Move comm primitives away from UnsafeArc

They currently still use `&mut self`, this migration was aimed towards moving
from UnsafeArc<T> to Arc<Unsafe<T>>
std: Remove UnsafeArc

This type has been superseded by Arc<Unsafe<T>>. The UnsafeArc type is a relic
of an era that has long since past, and with the introduction of liballoc the
standard library is able to use the Arc smart pointer. With little need left for
UnsafeArc, it was removed.

All existing code using UnsafeArc should either be reevaluated to whether it can
use only Arc, or it should transition to Arc<Unsafe<T>>

[breaking-change]
native: Remove UnsafeArc in favor of just Arc
rustuv: Remove usage of UnsafeArc
green: Remove usage of UnsafeArc
Add slice::MutableCloneableVector to the prelude

Every other trait in slice is in the prelude, so it makes sense to
provide MutableCloneableVector as well.
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`.
Update rust.vim for the latest prelude/keywords
vim: Handle box expressions specially

Attempt to highlight the placement expression in a `box (expr) foo`
expression. Also treat GC as a keyword within the placement expression.

This doesn't work correctly for arbitrary expressions. Notably, this
makes no attempt at balancing delimiters. I believe handling that will
require rewriting the syntax rules to add a region for every pair of
delimiters. That may be a desirable thing to do in the end, because we
can then rewrite our indent rules based on the syntax and get rid of
cindent(), but for the time being, we'll just live with the limitation.
green: Remove some unsafe code in BasicLoop
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 run-pass case for issue #14308

Enum wildcard patterns in match behave wrong when applied to tuple
structs. They either ICE or cause an LLVM error.
Fix ICE regarding tuple struct matches using (..)

Fixes half of #14308.