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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> @MaskRay Can you explain why this is gonna break musl? Thanks

+1; I think that information is basically what's holding this PR back from landing again. If we don't have specifics, I think it's fie to land the changes in a few days (after the branch happens!) to see if we can tease out the failure (and any other ones we're not aware of).

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