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

[Pierre d'Herbemont via cfe-commits]

pdherbemont wrote:

Ah nice catch! Thank you @aeubanks and @delcypher – I am going to try to understand why (and add a new test)

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