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

[via llvm-commits]

Sirraide wrote:

> Maybe the easiest way would be to add a real attribute for clspv?

What exactly would the requirements for such an attribute be? Does it need to take a parameter or do you only need one way of tagging functions? Asking because I can go ahead add one as part of this pr if that’s the easiest solution so we can clean up this `assume` situation. That also means we wouldn’t need an `omp_assume` spelling for the `assume` attribute. 

On that note, what name would you recommend for such an attribute? Probably `cl_something` or `clspv_something`, but I’m not sure what that `something` would be.

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