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

[Rainer Orth via cfe-commits]

rorth wrote:

Thanks.
 
FWIW, this patch
```
diff --git a/llvm/include/llvm/Support/Compiler.h b/llvm/include/llvm/Support/Compiler.h
--- a/llvm/include/llvm/Support/Compiler.h
+++ b/llvm/include/llvm/Support/Compiler.h
@@ -766,7 +766,8 @@ void AnnotateIgnoreWritesEnd(const char 
 /// Expands to true if runtime detection of SSE4.2 is supported.
 /// This can be used to guard runtime checks for SSE4.2 support.
 #if ((defined(__i386__) || defined(__x86_64__)) && defined(__has_attribute) && \
-     __has_attribute(target) && !defined(_WIN32))
+     __has_attribute(target) && !defined(_WIN32)) &&                           \
+    !(defined(__sun__) && defined(__svr4__))
 #define LLVM_SUPPORTS_RUNTIME_SSE42_CHECK 1
 #else
 #define LLVM_SUPPORTS_RUNTIME_SSE42_CHECK 0


```
also fixed the failures locally.

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