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

[Romaric Jodin via llvm-commits]

rjodinchr wrote:

This is the first time I'm trying to add an attribute, and I think I am missing something.
I am adding this part in `Attr.td`:
```
def ClspvLibclcBuiltin: DeclOrStmtAttr {
  let Spellings = [Clang<"clspv_libclc_builtin">];
  let Documentation = [Undocumented];
  let SimpleHandler = 1;
}
```
It allows the source to be parsed, but then I don't see the attribute in the LLVM IR generated for libclc.

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