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

[Thibault Monnier via cfe-commits]

Thibault-Monnier wrote:

Thanks for all the reports. I have confirmed the issue is due to this branch. I'll investigate / revert ASAP.

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