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

[Thibault Monnier via cfe-commits]

Thibault-Monnier wrote:

> Although this PR itself is insignificant, it will become a precedent - people might start adding runtime dispatch everywhere in LLVM - why not?

Is this really an issue? If we get better backwards-compatible performance, how is that bad? There's no reason to give distros a reason to switch to -march if we can do as well without it.

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