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

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

I mean, I'm not particularly attached to the syntax of the "if".  I guess we could designate `if (__builtin_amdgcn_processor_is("gfx900")) {}` as a "processor-feature-if". The point is that we need to know at the AST level which processor features are available for each statement.

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