[clang] [C23] Implement WG14 N3037 (PR #132939)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> > So the changes needed to move this patch forward are to only perform the attribute check when we're checking for a valid redefinition, not type compatibility in general. A follow-up to improve upon this can do the layout check when looking for compatibility. Alternatively (or as another follow-up), we can implement the case-by-case basis logic.
> > Does that match your understanding @jyknight?
> 
> Ping on this @jyknight

Another ping (if no response by mid-week, I'll probably land the changes as-is and address concerns post-commit).

https://github.com/llvm/llvm-project/pull/132939