[clang] [Clang][Lexer][Performance] Optimize Lexer whitespace skipping logic (PR #180819)

[Thibault Monnier via cfe-commits]

Thibault-Monnier wrote:

>From my previous tests, it was actually slower. I guess the table lookup ends up being predicted very well by the CPU, which allows it to not wait too much?

https://github.com/llvm/llvm-project/pull/180819