For E0277 on `for` loops, point at the "head" expression

When E0277's span points at a `for` loop, the actual issue is in the
element being iterated. Instead of pointing at the entire loop, point
only at the first line (when possible) so that the span ends in the
element for which E0277 was triggered.

Match libunwind's EABI selection with libpanic_unwind

Currently, the `libunwind` crate will only select the ARM EABI if it is compiling for ARM/Linux or Android targets. `libpanic_unwind`, however, will choose the ARM EABI if the target arch is ARM and the OS is not iOS. This means that if one tries to enable unwinding for a non-standard ARM target (such as implementing a custom stdlib via Xargo, for example), then the two crates can potentially disagree about which EABI is being targeted.

This PR makes `libunwind` use the [same logic](https://github.com/rust-lang/rust/blob/master/src/libpanic_unwind/gcc.rs#L139-L146) as `libpanic_unwind` when choosing the EABI.

I noticed there are a few comments about certain functions only differing on Android or ARM/Linux, but I *think* that those differences apply to the ARM EABI in general. Let me know if I'm wrong about that.

Modify the spans used for `for`-loop expression expansion, instead of
creating a new span during error creation.

Rollup of 19 pull requests

- Successful merges: #47415, #47437, #47439, #47453, #47460, #47502, #47529, #47600, #47607, #47618, #47626, #47656, #47668, #47696, #47701, #47705, #47710, #47711, #47719
- Failed merges: #47455, #47521

Print inlined functions on Windows

Split from https://github.com/rust-lang/rust/pull/45637

r? @alexcrichton

Fixes #47364

Update rls

Currently rls is missing from the latest nightly component. This PR brings it back.

We can't use git commands to compute a prerelease version when we're
building from a source tarball, or if git is otherwise unavailable.
We'll just call such builds `x.y.z-beta`, without a prerelease.

Looks like llvm 4.0.0 isn't read for that either.

Accidentally added in e9a64996

Add powerpc to run-pass/conditional-compile-arch.rs

Update compiler-builtins submodule

No sense of urgency, just wanted to make sure we don't lag too far behind!

First round of LLVM 6.0.0 compatibility

This includes a number of commits for the first round of upgrading to LLVM 6. There are still [lingering bugs](https://github.com/rust-lang/rust/issues/47683) but I believe all of this will nonetheless be necessary!

Fix ICE when use trees have multiple empty nested groups

The issue was caused by an oversight of mine in the original use_nested_groups PR, where different paths were resolved with the same `NodeId` in some cases (such as in `use {{}, {}};`).

Fixes #47673
r? @petrochenkov

Fixes for intra-doc-links

Turn errors into warnings, also handle methods, trait items, and variants.

r? @killercup @QuietMisdreavus

Make use of the implemented red/green algorithm for variance

r? @michaelwoerister

do not ICE when return type includes unconstrained anon region

It turns out that this *can* happen after all, if the region is only
used in projections from the input types.

Fixes https://github.com/rust-lang/rust/issues/47511

r? @EddyB

[perf] Use std based dedup in projection

Unstable sort was added recently, and the code that is being modified is 3 years old. As quicksort doesn't allocate it will likely perform as well as, or better than linear search.

I didn't benchmark. Have a perf run.