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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> Actually, I don't think we should use function multi-versioning here. x86-64 distributions that are aware of the performance should enable `-march=x86-64-v2` (roughly anything from Intel Nehalem 2008+ or AMD Bulldozer/Jaguar 2011+), which implies SSE4.2. Then the SSE4.2 code will be static.

Ideally, we would use multi-versioning here because that gets the best performance without making people performance tune the compiler when they build. This improves throughput for our pre- and post-commit CI pipeline, debug builds our developers are using, etc.

That said, I think it's reasonable to expect distributions and downstreams to do that performance tuning themselves if that's the only viable way forward at the moment.

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