rustc: Encode argument names for traits

This ensures that rustdoc can properly document inlined traits across crates.

Closes #14670
rustc: Avoid UB with signed division/remainder

Division and remainder by 0 are undefined behavior, and are detected at runtime.
This commit adds support for ensuring that MIN / -1 is also checked for at
runtime, as this would cause signed overflow, or undefined behvaior.

Closes #8460
librustc: Update AutoObject adjustment in writeback.
Add workaround for archive reading bug in LLDB.

LLDB contains a bug that makes it crash if an archive it reads
contains a file the name of which is exactly 16 bytes long. This
bug recently has made it impossible to debug Rust applications with
LLDB because some standard libraries triggered it indirectly:
For rlibs, rustc includes the LLVM bytecode in the archive, giving
it the extension ".bc.deflate". For liballoc (for example) this
results in the 16 character filename "alloc.bc.deflate", which is
bad.

This commit replaces the ".bc.deflate" suffix with
".bytecode.deflate" which itself is already longer than 16 bytes,
thus making sure that the bug won't be run into anymore.

The bug could still be run into with 14 character filenames because
then the .o files will trigger it. However, this is much more rare
and working around it would introduce more complexity than necessary
at the moment. It can always be done later on, if the need arises.

Fixes #14356.
rustc: Avoid 16-byte filenames in rlibs

In addition to avoiding 16-byte filenames with bytecode files, this commit also
avoids 16-byte filenames with object files pulled in from native libraries.
rustdoc: Inline static documentation across crates
Remove reference to ~str in documentation
Rename Iterator::len to count

This commit carries out the request from issue #14678:

> The method `Iterator::len()` is surprising, as all the other uses of
> `len()` do not consume the value. `len()` would make more sense to be
> called `count()`, but that would collide with the current
> `Iterator::count(|T| -> bool) -> unit` method. That method, however, is
> a bit redundant, and can be easily replaced with
> `iter.filter(|x| x < 5).count()`.
> After this change, we could then define the `len()` method
> on `iter::ExactSize`.

Closes #14678.

[breaking-change]
Implement Eq for HashSet and HashMap

Also fix documentation references to PartialEq.
url: encode small bytes correctly.

Previously, bytes less than 16 would be encoded as %X, rather than %XX,
since the output width was left to be automatic.
mk: Run doc tests with --cfg dox

There were a few examples in the macros::builtin module that weren't being run
because they were being #[cfg]'d out.

Closes #14697
rustc: Preserve reachable extern fns with LTO

All rust functions are internal implementation details with respect to the ABI
exposed by crates, but extern fns are public components of the ABI and shouldn't
be stripped. This commit serializes reachable extern fns to metadata, so when
LTO is performed all of their symbols are not stripped.

Closes #14500
Removing unused wrapper to libc::close.
doc: Turn off special features for rustdoc tests

These were only used for the markdown tests, and there's no reason they should
be distinct from the other tests.
rustdoc: Submit examples to play.rust-lang.org

This grows a new option inside of rustdoc to add the ability to submit examples
to an external website. If the `--markdown-playground-url` command line option
or crate doc attribute `html_playground_url` is present, then examples will have
a button on hover to submit the code to the playground specified.

This commit enables submission of example code to play.rust-lang.org. The code
submitted is that which is tested by rustdoc, not necessarily the exact code
shown in the example.

Closes #14654
Test fixes from the rollup
auto merge of #14712 : alexcrichton/rust/rollup, r=alexcrichton

Closes #14675 (rustc: Encode argument names for traits)
Closes #14681 (rustc: Avoid UB with signed division/remainder)
Closes #14682 (librustc: Update AutoObject adjustment in writeback.)
Closes #14683 (Avoid 16-byte filenames in rlibs)
Closes #14687 (rustdoc: Inline static documentation across crates)
Closes #14689 (Remove reference to ~str in documentation)
Closes #14692 (Rename Iterator::len to count)
Closes #14693 (Implement Eq for HashSet and HashMap)
Closes #14699 (url: encode small bytes correctly.)
Closes #14700 (rustdoc: Submit examples to play.rust-lang.org)
Closes #14701 (mk: Run doc tests with --cfg dox)
Closes #14710 (rustc: Preserve reachable extern fns with LTO)
Closes #14711 (Removing unused wrapper to libc::close.)