[clang] Support `guarded_by` attribute and related attributes inside C structs and support late parsing them (PR #95455)

[Pierre d'Herbemont via cfe-commits]

pdherbemont wrote:

> @pdherbemont Thanks for working on this LGTM. Please confirm that the comments that @aaronpuchert wanted have been added because I couldn't see them.
> 
> Once you've done that we can get this merged :)

Done!

https://github.com/llvm/llvm-project/pull/95455