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

[Erich Keane via cfe-commits]

erichkeane wrote:

OpenCL is "C" based, not C++.  However, C23 DID add namespacing to attributes, so if OpenCL can use C23, we should be able to make the spelling `[[omp::assume]]`.  That said, I DO see value for an `__attribute__` spelling, so I think `omp_assume` is probably an unfortunate necessity.  

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