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

[Dan Liew via cfe-commits]

https://github.com/delcypher requested changes to this pull request.

@pdherbemont thanks for working on this. It looks pretty good and its great to see the late parsing support I added gaining new users. I just have some nits about missing test cases.

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