[clang] [libclc] [llvm] [openmp] [Clang] `__attribute__((assume))` refactor (PR #84934)

[Erich Keane via cfe-commits]

erichkeane wrote:

> > the GNU `__attribute__((assume))` spelling is no longer diagnosed as a C++23 extension
> 
> @erichkeane @AaronBallman Just wanted to double-check whether this is fine because I committed this change after this pr was approved.

Yep, that makes sense to me.  Still LGTM.

https://github.com/llvm/llvm-project/pull/84934