Rollup of 7 pull requests

Successful merges:

 - rust-lang#127679 (Stabilize `raw_ref_op` (RFC 2582))
 - rust-lang#128084 (Suggest adding Result return type for associated method in E0277.)
 - rust-lang#128628 (When deduplicating unreachable blocks, erase the source information.)
 - rust-lang#128902 (doc: std::env::var: Returns None for names with '=' or NUL byte)
 - rust-lang#129048 (Update `crosstool-ng` for loongarch64)
 - rust-lang#129116 (Include a copy of `compiler-rt` source in the `download-ci-llvm` tarball)
 - rust-lang#129208 (Fix order of normalization and recursion in const folding.)

r? `@ghost`
`@rustbot` modify labels: rollup

Rollup merge of rust-lang#129116 - Zalathar:compiler-rt, r=Mark-Simulacrum

Include a copy of `compiler-rt` source in the `download-ci-llvm` tarball

This will make it possible to experiment with allowing `download-ci-llvm` builds to build `library/profiler_builtins`, without needing to check out the `src/llvm-project` submodule.

By itself, this PR just adds the files to the tarball, but doesn't actually do anything with them. The idea is that once this is merged, it will then be much easier to proceed with work on the necessary bootstrap changes (using the real downloaded tarball), without having to rig up weird hacks to simulate downloading a modified tarball.

---

Adding these files to the compressed tarballs appears to increase its size by a negligible amount (<1 MB out of 400/800+ MB).

The uncompressed size is about 14 MB (out of 2+ GB for the whole tarball).

(The excluded test files would have been another 35 MB.)

Build `library/profiler_builtins` from `ci-llvm` if appropriate

Running all of `tests/coverage` requires the LLVM profiler runtime, which requires setting `build.profiler = true`.

Historically, doing that has required checking out the entire `src/llvm-project` submodule. For compiler contributors who otherwise don't need that submodule (thanks to `download-ci-vm`), that's quite inconvenient.

However, thanks to rust-lang#129116, the downloaded CI LLVM tarball now contains a copy of LLVM's `compiler-rt` directory, which includes all the files needed to build the profiler runtime. So with a little bit of extra logic in bootstrap, we can have `library/profiler_builtins` look for the `compiler-rt` files in `ci-llvm` instead of the `src/llvm-project` submodule.

Build `library/profiler_builtins` from `ci-llvm` if appropriate

Running all of `tests/coverage` requires the LLVM profiler runtime, which requires setting `build.profiler = true`.

Historically, doing that has required checking out the entire `src/llvm-project` submodule. For compiler contributors who otherwise don't need that submodule (thanks to `download-ci-vm`), that's quite inconvenient.

However, thanks to rust-lang#129116, the downloaded CI LLVM tarball now contains a copy of LLVM's `compiler-rt` directory, which includes all the files needed to build the profiler runtime. So with a little bit of extra logic in bootstrap, we can have `library/profiler_builtins` look for the `compiler-rt` files in `ci-llvm` instead of the `src/llvm-project` submodule.

Build `library/profiler_builtins` from `ci-llvm` if appropriate

Running all of `tests/coverage` requires the LLVM profiler runtime, which requires setting `build.profiler = true`.

Historically, doing that has required checking out the entire `src/llvm-project` submodule. For compiler contributors who otherwise don't need that submodule (thanks to `download-ci-vm`), that's quite inconvenient.

However, thanks to rust-lang#129116, the downloaded CI LLVM tarball now contains a copy of LLVM's `compiler-rt` directory, which includes all the files needed to build the profiler runtime. So with a little bit of extra logic in bootstrap, we can have `library/profiler_builtins` look for the `compiler-rt` files in `ci-llvm` instead of the `src/llvm-project` submodule.