This is a way to shrink call spans that doesn't involve mixing different spans,
and avoids overlap with argument spans.
This patch also removes some low-value comments that were causing rustfmt to
ignore the match arms.
Functions currently can't have mappings in multiple files, and if that ever
changes (e.g. to properly support expansion regions), this code will need to be
completely overhauled anyway.
As more and more of the span refiner's functionality has been pulled out into
separate early passes, it has finally reached the point where we can remove the
rest of the old `SpansRefiner` code, and replace it with a single
modestly-sized function.
Some of these cases currently don't occur in practice, but are included for
completeness, and to avoid having to add them later as branch coverage and
MC/DC coverage start building more complex expressions.
