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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

LGTM, I can't think of a better way to express this. Thank you for all your efforts on it!

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