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

[Pierre d'Herbemont via cfe-commits]

pdherbemont wrote:

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

They already work fine in C so far. But if you are aware of some issues don't hesitate to let me know!

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