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

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Sirraide wrote:

> However, I'd suggest instead adding the clspv attribute in a separate review/commit. It isn't really related to this one, other than this has a slight dependency on it.

I could add that in a separate pr; that means that one will have to be merged before this one because we have to replace that one use of `assume` in the clcfunc.h header with the new attribute either before or at the same time as we remove the `assume` spelling.

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