[clang] [Clang][Lexer] Reland "Detect SSE4.2 availability at runtime in fastParseASCIIIdentifier" (PR #175452)

[Thibault Monnier via cfe-commits]

Thibault-Monnier wrote:

Yes, as requested by Cor3ntin because it added a lot of preprocessor complexity for almost no runtime impact. The support check is only done once.

https://github.com/llvm/llvm-project/pull/175452