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

[Pierre d'Herbemont via cfe-commits]

pdherbemont wrote:

> @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.

I should have added all the requested test cases. Not sure it's the way you wanted it, so let me know if you want any change!

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