Improve EXPLAIN's ability to cope with LATERAL references in plans.

Improve EXPLAIN's ability to cope with LATERAL references in plans.

push_child_plan/pop_child_plan didn't bother to adjust the "ancestors"
list of parent plan nodes when descending to a child plan node.  I think
this was okay when it was written, but it's not okay in the presence of
LATERAL references, since a subplan node could easily be returning a
LATERAL value back up to the same nestloop node that provides the value.
Per changed regression test results, the omission led to failure to
interpret Param nodes that have perfectly good interpretations.