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

[Corentin Jabot via cfe-commits]

https://github.com/cor3ntin approved this pull request.

Sorry I did not see your earlier ping.
Feel free to ping reviewers weekly in the future.

The whole situation with ifuncs makes me sad....

Let's go with this.

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