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

[Romaric Jodin via cfe-commits]

rjodinchr wrote:

> The libclc usage seems to have been added by @rjodinchr in https://reviews.llvm.org/D147773 (and approved by @alan-baker). Perhaps they have an opinion?

https://github.com/google/clspv uses the assume attribute. The goal is to be able to put a mark on libclc functions to have a specific lowering in `clspv`.
I'm not against removing the attribute, but we will have to find another way to do that for `clspv`. Do you know something we could use to get rid of the assume attribute?

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