Teach convert_subquery_pathkeys() to handle the case where the

Teach convert_subquery_pathkeys() to handle the case where the
subquery's pathkey is a RelabelType applied to something that appears
in the subquery's output; for example where the subquery returns a
varchar Var and the sort order is shown as that Var coerced to text.
This comes up because varchar doesn't have its own sort operator.
Per example from Peter Hardman.