Previously, we would retrieve the span from the `Body` using
the `locations` field. However, we may end up changing the
`locations` field when moving a constraint from a promoted
to a different body.
We now store the original `Span` in a dedication field, so that
changes to the `locations` do not affect the quality of our
diagnostics.
In two cases where this ordering was used, I've replaced the sorting
to use a key that does not include DefId. I'm not sure this is correct
in terms of our goals from #90317, or otherwise.
