[clang] [Clang] [Lexer] Detect SSE4.2 availability at runtime in fastParseASCIIIdentifier (PR #171914)

[via cfe-commits]

serge-sans-paille wrote:

@Thibault-Monnier there would still be a runtime check (through `ifunc`, see https://godbolt.org/z/T156T75Ef).
You can remove this at the cost of more macro guard, something along those lines https://godbolt.org/z/fKcc9qjdc 

https://github.com/llvm/llvm-project/pull/171914