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

[via llvm-commits]

Sirraide wrote:

> @Sirraide, would you add those (or similar if you feel that changes are needed) directly to that PR?

Yeah, that looks reasonable. If this works for clspv, then I’ll integrate these changes into this pr.

That means I can remove the `omp_assume` spelling for `[[omp::assume]]` that this pr was going to add as a temporary measure, because that’s not going to be needed anymore.

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