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

[Yeoul Na via cfe-commits]

rapidsna wrote:

> > You may also want to consider making the attribute late parsed in C when `-fexperimental-late-parse-attributes` is enabled. See https://github.com/llvm/llvm-project/pull/93121/files#diff-ae2ec9524bdbeea1f06917607482634dd89af5bcbb929805032463e5dafe79e7R2260
> > That will allow the code like below:
> > ```
> >  struct Foo {
> >    int a_value GUARDED_BY(mu_); // attribute comes before `mu_` which needs to be late parsed
> >    struct Mutext *mu_;
> >  }
> > ```
> 
> Adopted `LateAttrParseExperimentalExt`. Let me know if that looks okay.

Thanks! Could you please also add a test taking advantage of late parsing?

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