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

[Rainer Orth via cfe-commits]

rorth wrote:

It seems that this patch is mostly based on GCC's `libcpp/lex.cc`, which has this comment:
```
/* Disable on Solaris 2/x86 until the following problem can be properly
   autoconfed:

   The Solaris 10+ assembler tags objects with the instruction set
   extensions used, so SSE4.2 executables cannot run on machines that
   don't support that extension.  */

#if (GCC_VERSION >= 4005) && (__GNUC__ >= 5 || !defined(__PIC__)) && (defined(__i386__) || defined(__x86_64__)) && !(defined(__sun__) && defined(__svr4__))
```
While the current issue is different (since the `clang` integrated assembler is used, no Solaris hardware capabilities are created, unlike with the native assembler), as a stopgap measure `LLVM_SUPPORTS_RUNTIME_SSE42_CHECK` could be disabled on Solaris in a similar way.

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