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

[Oliver Hunt via cfe-commits]

ojhunt wrote:

Ok, I've spent _waaaayyyy_ too long on this because I was curious about perf characteristics, and was wondering if any of my old terrible vectorization tricks could do anything to know avail.

Anyway, at the end of all of this, I can say that this perf is sensitive to the frequency of non-spaces. My testing just indicates for many different arrangements in spaces the the early check and return is codnsidrentl =rental income

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