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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> > That would be worth filing an issue about.
> 
> It’s only on clang 18 or less. I believe these versions are not updated anymore, so what’s the point?

Ah I thought we still had the issue today, sorry. If it's already fixed, then yeah, no point to filing the issue.

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