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

[Aaron Ballman via cfe-commits]

================
@@ -1943,14 +1956,22 @@ fastParseASCIIIdentifier(const char *CurPtr,
       continue;
     return CurPtr;
   }
-#endif
 
-  unsigned char C = *CurPtr;
-  while (isAsciiIdentifierContinue(C))
-    C = *++CurPtr;
-  return CurPtr;
+  return fastParseASCIIIdentifierScalar(CurPtr);
 }
 
+#ifndef __SSE4_2__
+__attribute__((target("default")))
+#endif
+#endif
+#ifndef __SSE4_2__
+static const char *
+fastParseASCIIIdentifier(const char *CurPtr,
+                         [[maybe_unused]] const char *BufferEnd) {
----------------
AaronBallman wrote:

```suggestion
                         const char *) {
```
This makes it more clear it's always unused.

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