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

[Pierre d'Herbemont via cfe-commits]

pdherbemont wrote:

> LGTM assuming you will be fixing the buildkite failures.
> 
> You may still need to wait on @AaronBallman's approval.

Fixed the buildkite issue – they were regression from the latest changes about acquired_{before,after} handling. Also pinged @AaronBallman. Thank you all for the feedback!

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