Add an explicit `Span` field to `OutlivesConstraint`
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.
463c94a780