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

[Thibault Monnier via cfe-commits]

Thibault-Monnier wrote:

> You have links to the previous failure? I find it concerning that we would not be able to use ifuncs.

@cor3ntin it seems that @MaskRay was able to find a "bug" in glibc that explains our issues—thank you for the insight : https://github.com/llvm/llvm-project/pull/175452#issuecomment-4901399292.

I'm afraid this means there is no way to move forward with ifuncs if we want to remain backwards-compatible (unless I misunderstand something which is totally possible). 

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