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

[Thibault Monnier via cfe-commits]

Thibault-Monnier wrote:

> I think llvm/clang as open-source compiler should encourage users who are aware of performance to use more advanced -march.

@Andarwinux 
Most people do not compile their own version of Clang (and many distros prefer to remain backwards-compatible). The whole piont of this PR is to allow the performance gain to all users who support this feature rather than those who put their hands on a -march binary. You can still use -march which should (though MaskRay said otherwise?) not cause any regressions, so this seems like a net improvement to me.

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