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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> > One question I have is whether there will be follow-up work for other thread safety attributes (`acquire_capability`, `try_acquire_capability`, etc)?
> 
> They already work fine in C so far. But if you are aware of some issues don't hesitate to let me know!

Ah, sorry, of course I picked examples that appertain to functions rather than fields. But some of the other thread safety attributes have the same problem as `guarded_by`: https://godbolt.org/z/hWTMxzrns

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