[clang] [clang] Optimize Lexer hot path to reduce compile time (PR #177153)

[Nikita Popov via cfe-commits]

nikic wrote:

Compile-time: https://llvm-compile-time-tracker.com/compare.php?from=680124ca9a06bd7413cf4cc1dab80f929c7d0bca&to=00edb53c7e3e79eeab95f4f4990c6d9041cd2a76&stat=instructions:u

This doesn't recover the original regression, but is an improvement.

https://github.com/llvm/llvm-project/pull/177153