[clang] [llvm] [AMDGPU][clang][CodeGen][opt] Add late-resolved feature identifying predicates (PR #134016)

[Alex Voicu via llvm-commits]

AlexVlx wrote:

> 1. Have what is proposed here as an initial step, with the addition that we issue warnings on unguarded uses of builtins / ASM (similar to what `__builtin_available` / `@available` do), and we clean-up non-extern functions that become unreachable as a consequence of predicate expansion (i.e. `foo` can only be called from within this module, and it was only being called from a predicate guarded block, which was removed);

Gentle ping given that the above has been added, with the caveat that warning on unguarded ASM hasn't been added yet, and the diagnostics are conservative & optimistic (we assume that if a guard exists it is correct, and do not do a feature check). This will be addressed in a subsequent patch which builds on what is now there, because we need some additional internal discussion on the AMD-side around the shape of these diagnostics, and because it would add some more girth to what is already a pretty large change.

https://github.com/llvm/llvm-project/pull/134016