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

[Mikael Holmen via cfe-commits]

mikaelholmen wrote:

> Yes, I removed the caller and forgot to update the name.

Ok, well if I change ```fastParseASCIIIdentifierSSE42``` to ```fastParseASCIIIdentifier``` I still get
```
../../clang/lib/Lex/Lexer.cpp:1937:1: error: unused function 'fastParseASCIIIdentifier' [-Werror,-Wunused-function]
 1937 | fastParseASCIIIdentifier(const char *CurPtr, const char *BufferEnd) {
      | ^~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.
```
so the optimized version still looks unused.

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