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

[John McCall via llvm-commits]

rjmccall wrote:

> Slightly independently, cpu_supports might turn out a bit difficult to use, at least for us (and I suspect other targets), as the feature definitions are often ad hoc and sadly mutable.

Hmm. Well, you get to define what feature names you recognize in `__builtin_cpu_supports`, so there's no reason to use marketing names, and if they're unstable you probably shouldn't. But I would imagine there's *some* stable technical grouping that's coarser-grained than whether an individual builtin is available.

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