Ensure that the phrels sets of PlaceHolderVars appearing in an AppendRelInfo's

Ensure that the phrels sets of PlaceHolderVars appearing in an AppendRelInfo's
translated_vars list get updated when pulling up an appendrel member.  It's
not clear that this really matters at present, since relatively little gets
done with the outputs of an appendrel child relation; but it probably will
come back to bite us sometime if we leave them with the wrong values.