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

[Mikael Holmen via cfe-commits]

mikaelholmen wrote:

> so the optimized version still looks unused.

I get the warning with clang 18.1.8 which is what we usually build with, but I noticed now that I don't seem to get the warning with clang 19.1.6.

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