I discovered that `impl Debug for str` is quite slow because it ends up doing a `unicode_data::grapheme_extend::lookup` for each char, which ends up doing a binary search.

This introduces a fast-path for ASCII chars which do not have this property.

The `lookup` is thus completely gone from profiles.

This is so that we can catch stray test output files, since test names
without dots can form predictable patterns we can match on.

Add ASCII fast-path for `char::is_grapheme_extended`

I discovered that `impl Debug for str` is quite slow because it ends up doing a `unicode_data::grapheme_extend::lookup` for each char, which ends up doing a binary search.

This introduces a fast-path for ASCII chars which do not have this property.

The `lookup` is thus completely gone from profiles.

---

As a followup, maybe it’s worth implementing this fast path directly in `unicode_data` so that it can check for the lower bound directly before going to a potentially expensive binary search.

…zkan

tidy: split dots in filename not the entire path when checking for stray stdout/stderr files

I committed a path crime by splitting the entire path on `.`, when I meant to split on the filename. This means that any parent folders which contain `.` will cause tidy failure. Added a regression test so that doesn't happen again.

### Follow-up

- [ ] Adjust rustc-dev-guide to document assert on test name not containing dots. rust-lang/rustc-dev-guide#1927

Fixes #121986.