[llvm] [llvm] Improve implementation of StringRef::find_last_of and cie (PR #71865)

[via llvm-commits]

serge-sans-paille wrote:

> This patch looks good to me. I am not sure if we want to implement architecture-specific optimizations like `__SSE2__` at the source-code level. 

We already have these for critical part of the code (in the clang lexer for instance). I only went that way because this particular function appeared in a profile of a teamate's machine during code indexing (and because it was very entertaining to implement).

> EDIT: I guess "this patch" isn't clear. I am referring to the first one that replaces `"."` with `'.'`, etc.

I've landed the first commit as 33b51588c5d51995909f1f0be0b47b1708dac4fa, it was a no-brainer

https://github.com/llvm/llvm-project/pull/71865