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

[Yeoul Na via cfe-commits]

https://github.com/rapidsna approved this pull request.

LGTM assuming you will be fixing the buildkite failures. 

You may still need to wait on @AaronBallman's approval. 

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