[clang] Support [[guarded_by(mutex)]] attribute inside C struct (PR #94216)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

Thank you for working on this!

The changes should come with a release note in clang/docs/ReleaseNotes.rst so users know about the new functionality.

One question I have is whether there will be follow-up work for other thread safety attributes (`acquire_capability`, `try_acquire_capability`, etc)?

https://github.com/llvm/llvm-project/pull/94216