[clang] [clang] Change representation of CurLexerKind (PR #70381)

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serge-sans-paille wrote:

Updated compile time metrics: https://llvm-compile-time-tracker.com/compare.php?from=f9906508bc4f05d3950e2219b4c56f6c078a61ef&to=2631a5163de552a8e482f5cbc51525fb8bb5082f&stat=instructions:u

https://github.com/llvm/llvm-project/pull/70381