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

[Thibault Monnier via cfe-commits]

Thibault-Monnier 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?


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