This improves error messages by indicating when slices above a certain
lengths have not been matched. Previously, we would only report examples
of such lengths, but of course never all of them.
This is one of the behaviors we no longer allow in NLL. Since it can
lead to undefined behavior, I think it's definitely worth making it a
hard error without waiting to turn off migration mode (#58781).
Closes#60450.
My ulterior motive here is making it impossible to leave variables
partially initialized across a yield (see discussion at #63035), so
tests are included for that.
