Fix parent of WCO qual.

Fix parent of WCO qual.

The parent of some WCO expressions was, apparently by accident, set to
the the source of DML queries, rather than the target table.  This
causes problems for the upcoming pluggable storage work, because the
target and source table might be of different storage types.

It's possible that this is already problematic, but neither
experimenting nor inquiries on -hackers have found them. So don't
back for now.

Author: Andres Freund
Discussion: https://postgr.es/m/20181205225213[email protected]