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

[Pierre d'Herbemont via cfe-commits]

pdherbemont 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.

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