Teach `compiletest` to ignore platform triples

The UI tests are written assuming `--remap-path-prefix` is *not used* (`remap-debuginfo` in `config.toml`). The consequence is that the error messages may include paths and snippets into the standard library. When `remap-debuginfo` is enabled, these messages change in format and structure because `rustc` will not show paths and snippets into the standard library.

This normally isn't a problem for the "main" platforms (linux/macos/windows), because the CI infrastructure is set up so that the tests run without `remap-debuginfo`, but the `dist` artifacts are built separately with `remap-debuginfo` enabled. However, some of the lower-tier platforms perform both tests and distribution in a single step with `remap-debuginfo` enabled. This also affects developers and distributors who use `remap-debuginfo`.

To sidestep this problem, we add a way to ignore tests in specific platform triples, and update the overly broad `ignore-x86` rule in affected tests.

Address #46948, #54546, #53081.

doc comments: Less attribute mimicking

Make sure doc comments are not converted into intermediate meta-items, or not mixed with `doc(inline)` or something like that.

Follow-up to https://github.com/rust-lang/rust/pull/65750.

prune ill-conceived BTreeMap iter_mut assertion and test its mutability

Proposal to deal with #67438 (and I'm more sure now that this is the right thing to do).
Passes testing with miri.

update miri

tidy: change msdn links to newer locations

see accouncement at https://docs.microsoft.com/welcome-to-docs
The script that I used: https://gist.github.com/lzutao/1449c9210ad91899841d62e0058d2caa

Fix ICE / miscompilation when inlining simd shuffle intrinsic in MIR.

Closes #67557.

r? @oli-obk

Rollup of 15 pull requests

Successful merges:

 - #65244 (add IntoFuture trait and support for await)
 - #67576 (reuse `capacity` variable in slice::repeat)
 - #67588 (Use NonNull in slice::Iter and slice::IterMut.)
 - #67594 (Update libc to 0.2.66)
 - #67602 (Use issue = "none" instead of "0" in intrinsics)
 - #67604 (Add Scalar::to_(u|i)16 methods)
 - #67617 (Remove `compiler_builtins_lib` documentation)
 - #67621 (Use the correct type for static qualifs)
 - #67629 (Remove redundant link texts)
 - #67632 (Convert collapsed to shortcut reference links)
 - #67633 (Update .mailmap)
 - #67635 (Document safety of Path casting)
 - #67654 (Add regression test for old NLL ICE)
 - #67659 (Stabilize the `matches!` macro)
 - #67664 (Fix some mailmap entries)

Failed merges:

r? @ghost

Fix some mailmap entries

r? @pietroalbini

Stabilize the `matches!` macro

Fixes https://github.com/rust-lang/rust/issues/65721

FCP: https://github.com/rust-lang/rust/issues/65721#issuecomment-569118119

Add regression test for old NLL ICE

This fails on nightly-2018-06-24.

Resolves #51770

Document safety of Path casting

I would personally feel more comfortable making the relevant (internal anyway) types repr(transparent) and then documenting that we can make these casts because of that, but I believe this is a more minimal PR, so posting it first.

Resolves #45910.

Update .mailmap

Convert collapsed to shortcut reference links

Remove redundant link texts

Most of these links are followed by a parenthesized expression.  I think that the redundant link texts were added to prevent interpretation as an inline link.  This is unnecessary since the closing square bracket and opening parenthesis are separated by whitespace.

Use the correct type for static qualifs

Closes #67609

Remove `compiler_builtins_lib` documentation

Fixes #67593

Add Scalar::to_(u|i)16 methods

r? @RalfJung

Use issue = "none" instead of "0" in intrinsics

`issue = "0"` was incorrectly used in https://github.com/rust-lang/rust/commit/9ee16e14c51668309f446636e8960b7fbbac066d, the project is moving away from issue 0

Update libc to 0.2.66

cc @gnzlbg @alexcrichton

Use NonNull in slice::Iter and slice::IterMut.

`ptr` of `slice::Iter` and `slice::IterMut` can never be null, but this
fact wasn't exploited for layout optimizations. By changing `ptr` from
`*<mutability> T` to `NonNull<T>`, the compiler can now optimize layout
of `Option<Iter<'a, T>>`.

Closes #67228

reuse `capacity` variable in slice::repeat

None

add IntoFuture trait and support for await

The [async-await RFC](https://rust-lang.github.io/rfcs/2394-async_await.html#the-await-compiler-built-in) mentions being able to `await` anything implementing `IntoFuture`. Somewhere along the way, it was left out.

Implement padding for IpAddr without heap alloc

Implements padding for `IpAddr`s without heap allocations.
This fixes issue #66810 .

cc @jethrogb @mzohreva

Fixes https://github.com/rust-lang/rust/issues/65721

FCP: https://github.com/rust-lang/rust/issues/65721#issuecomment-569118119

Add LLVM `skip-rebuild` option to `x.py`

This PR reimplements parts of @Walther's work from #65848, and closes #65612.

I decided not to implement the [arguments to override this setting](https://github.com/rust-lang/rust/issues/65612#issuecomment-544247546) in this PR. If there's strong feeling that this change shouldn't be merged without the overrides then I'm happy to close this until I've had a chance to add them in. Otherwise I'll aim to submit a second PR with those this weekend.

I'd have liked to have tested the change in `native.rs`, but there didn't seem to be any existing test infrastructure. I ran this a few times manually and it _worked on my machine_ though... 😬

Various const eval and pattern matching ICE fixes

r? @RalfJung
cc @spastorino

This PR does not change existing behaviour anymore and just fixes a bunch of ICEs reachable from user code (sometimes even on stable via obscure union transmutes).

Allocate HIR on an arena 2/4 -- Expr & Pat

This is the second PR in the series started by #66931

This time, commits don't really make sense on their own.
They are mostly split by type of compile error.

The additional diff is here: https://github.com/cjgillot/rust/compare/hirene-preamble...hirene-expr