[clang] [llvm] [Clang][Lexer] Reland "Detect SSE4.2 availability at runtime in fastParseASCIIIdentifier" (PR #180631)

[Corentin Jabot via cfe-commits]

cor3ntin wrote:

@MaskRay we are getting a lot of mileage out of the small amount of arch-specific we have (ascii identifier, comments) are really the only two places where we do that. I would not be opposed to extending that support to other archs (arm in particular)

https://github.com/llvm/llvm-project/pull/180631