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

[Thibault Monnier via llvm-commits]

Thibault-Monnier wrote:

> We did switch to use LLVM's `llvm-libtool-darwin` in stage2 build now and we no longer see any assertion error.

Note that I reverted in the meantime, could that be the reason?

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