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

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Andarwinux wrote:

But that would lead to binary bloat, since it would contain two versions anyway, right? Though not significant in this case, I'm afraid it opens up a Pandora's box.

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