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

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cor3ntin wrote:

@serge-sans-paille I'm still trying to understand whether this change will be tested by CI - or at all.
SSE4 is not enabled by default and i don't think our cmake does that either - I did ping @AaronBallman and we are still thinking about it

https://github.com/llvm/llvm-project/pull/68962