[clang] Provide an SSE4.2 implementation of identifier token lexer (#68962)

The _mm_cmpistri instruction can be used to quickly parse identifiers.

With this patch activated, clang pre-processes <iostream> 1.8% faster,
and sqlite3.c amalgametion 1.5% faster, based on time measurements and
number of executed instructions as measured by valgrind.

The introduction of an extra helper function in the regular case has no
impact on performance, see


https://llvm-compile-time-tracker.com/compare.php?from=30240e428f0ec7d4a6d1b84f9f807ce12b46cfd1&to=12bcb016cde4579ca7b75397762098c03eb4f264&stat=instructions:u



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Co-authored-by: serge-sans-paille <sguelton@mozilla.com>