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

[Pierre d'Herbemont via cfe-commits]

pdherbemont wrote:

Pushed an additional change on the documentation: the doc was incorrectly stating that ACQUIRED_{AFTER,BEFORE} were not implemented but they are.

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