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

[Corentin Jabot via llvm-commits]

cor3ntin wrote:

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

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