ship rust analyzer

This successfully  builds rust-analyzer as a part of rust repo. I haven't yet added required changes to dist.rs -- seems like I just have to copy-paste quite a bit of code I don't really understand :-)

Rollup of 12 pull requests

Successful merges:

 - #73140 (Fallback to xml.etree.ElementTree)
 - #73670 (Add `format_args_capture` feature)
 - #73693 (Use exhaustive match in const_prop.rs)
 - #73845 (Use &raw in A|Rc::as_ptr)
 - #73861 (Create E0768)
 - #73881 (Standardize bibliographic citations in rustc API docs)
 - #73925 (Improve comments from #72617, as suggested by RalfJung)
 - #73949 ([mir-opt] Fix mis-optimization and other issues with the SimplifyArmIdentity pass)
 - #73984 (Edit docs for rustc_data_structures::graph::scc)
 - #73985 (Fix "getting started" link)
 - #73997 (fix typo)
 - #73999 (Bump mingw-check CI image from Ubuntu 16.04 to 18.04.)

Failed merges:

 - #74000 (add `lazy_normalization_consts` feature gate)

r? @ghost

Bump mingw-check CI image from Ubuntu 16.04 to 18.04.

I chose 18.04 because we use it for other builders, and it's enough to get a version of MinGW that can build `libssh2-sys`.

This is a prereq for #73902, where `libssh2-sys` shows up as an indirect dependency of `x.py check src/tools/semverver` (through `src/tools/cargo`, which we don't currently `x.py check` because it's not in-tree). See also https://github.com/rust-lang/rust/pull/73902#issuecomment-652414502.

r? @Mark-Simulacrum cc @mati865

fix typo

r? @Dylan-DPC

Fix "getting started" link

The previous link is 404.

Edit docs for rustc_data_structures::graph::scc

- Add newline to provide concise module summary
- Add wikipedia link
- Italicize O notation

[mir-opt] Fix mis-optimization and other issues with the SimplifyArmIdentity pass

This does not yet attempt re-enabling the pass, but it does resolve a number of issues with the pass.

r? @oli-obk

I believe this closes #73223.

Improve comments from #72617, as suggested by RalfJung

r? @RalfJung

Standardize bibliographic citations in rustc API docs

See #73877.

Create E0768

r? @Dylan-DPC

Use &raw in A|Rc::as_ptr

This PR uses `&raw` for offsetting `*mut [A]RcInner<T> -> *mut T`.

Additionally, this updates the implementation of `Weak::as_ptr` to support unsized `T`, though it does not yet relax the bounds of `Weak::as_ptr`/`into_raw`/`from_raw` to accept unsized `T`.

Use exhaustive match in const_prop.rs

Addresses a comment left by @RalfJung on #73613

r? @RalfJung

Add `format_args_capture` feature

This is the initial implementation PR for [RFC 2795](https://github.com/rust-lang/rfcs/pull/2795).

Note that, as dicussed in the tracking issue (#67984), the feature gate has been called `format_args_capture`.

Next up I guess I need to add documentation for this feature. I've not written any docs before for rustc / std so I would appreciate suggestions on where I should add docs.

Fallback to xml.etree.ElementTree

The xml.etree.cElementTree has been deprecated since Python 3.3
and removed in Python 3.9 https://bugs.python.org/issue36543.

Add Docker image to run AArch64 Linux tests

This PR adds a Docker image to run the AArch64 Linux test suite on a native AArch64 host platform, which will be used in the future to run the test suite in our CI. The image will also be useful for ARM folks to ensure internally that the bugfixes they submit work.

This will be the first Docker image designed to run on a non-x86_64 host platform, and to prevent surprising behavior this PR moves all images requiring a x86_64 host in the `src/ci/docker/host-x86_64` directory. Paths and scripts are changed accordingly, and a helpful error message is added when someone tries to run an image on the wrong architecture:

```
Invalid image: aarch64-gnu
Note: the image exists for the aarch64 host architecture
Note: the current host architecture is x86_64
```

The old emulated and disabled `aarch64-gnu` builder is also removed in this PR.

This PR is best reviewed commit-by-commit.
r? @Mark-Simulacrum

- Add newline to provide concise module summary
- Add wikipedia link
- Italicize O notation

The current plan is that submodule tracks the `release` branch of
rust-analyzer, which is updated once a week.

rust-analyzer is a workspace (with a virtual manifest), the actual
binary is provide by `crates/rust-analyzer` package.

Note that we intentionally don't add rust-analyzer to `Kind::Test`,
for two reasons.

*First*, at the moment rust-analyzer's test suite does a couple of
things which might not work in the context of rust repository. For
example, it shells out directly to `rustup` and `rustfmt`. So, making
this work requires non-trivial efforts.

*Second*, it seems unlikely that running tests in rust-lang/rust repo
would provide any additional guarantees. rust-analyzer builds with
stable and does not depend on the specifics of the compiler, so
changes to compiler can't break ra, unless they break stability
guarantee. Additionally, rust-analyzer itself is gated on bors, so we
are pretty confident that test suite passes.

The previous link is 404.

We're starting to include native aarch64 machines in our CI, but before
this commit the architecture wasn't included in the cache key for our
Docker images. This means there could be conflicts between images
produced on different architectures, hurting our CI times.

This commit fixes the problem by including the output of `uname -m` in
the cache key.

We need to add runners designed for an aarch64 host system, and it'd be
nice to return an error message if someone tries to run an image
designed for an host architecture in another one.

To start the work on this, this commit moves all the existing builders
in the host-x86_64 directory, and changes the run.sh script to look up
the image in the correct directory based on the host architecture.

Update cargo, rls

## cargo
14 commits in c26576f9adddd254b3dd63aecba176434290a9f6..fede83ccf973457de319ba6fa0e36ead454d2e20
2020-06-23 16:21:21 +0000 to 2020-07-02 21:51:34 +0000
- Fix overflow error on 32-bit. (rust-lang/cargo#8446)
- Exclude the target directory from backups using CACHEDIR.TAG (rust-lang/cargo#8378)
- CONTRIBUTING.md: Link to Zulip rather than Discord (rust-lang/cargo#8436)
- Update built-in help for features (rust-lang/cargo#8433)
- Update core-foundation requirement from 0.7.0 to 0.9.0 (rust-lang/cargo#8432)
- Parse `# env-dep` directives in dep-info files (rust-lang/cargo#8421)
- Move string interning to util (rust-lang/cargo#8419)
- Expose built cdylib artifacts in the Compilation structure (rust-lang/cargo#8418)
- Remove unused serde_derive dependency from the crates.io crate (rust-lang/cargo#8416)
- Remove unused remove_dir_all dependency (rust-lang/cargo#8412)
- Improve git error messages a bit (rust-lang/cargo#8409)
- Improve the description of Config.home_path (rust-lang/cargo#8408)
- Improve support for non-`master` main branches (rust-lang/cargo#8364)
- Document that OUT_DIR in JSON messages is an absolute path (rust-lang/cargo#8403)

## rls
2020-06-19 15:36:00 +0200 to 2020-06-30 23:34:52 +0200
- Update cargo (rust-lang-nursery/rls#1686)

Avoid `unwrap_or_else` in `RawVec::allocate_in`.

This reduces the amount of LLVM IR generated by up to 1 or 2%.

r? @Amanieu

If temporaries are used beyond just the temporary chain, then we can't
optimize out the reads and writes.

Rollup of 8 pull requests

Successful merges:

 - #73454 (Move contributing.md to rustc-dev-guide and point at getting started)
 - #73724 (Use WASM's saturating casts if they are available)
 - #73726 (resolve: disallow labelled breaks/continues through closures/async blocks)
 - #73753 (Use 'tcx for references to AccessLevels wherever possible.)
 - #73781 (Update psm version)
 - #73952 (Add option for local docker testing.)
 - #73957 (disable BTree min_max test in Miri for now)
 - #73975 (Document rustc_ast::ast::Pat)

Failed merges:

r? @ghost