Store information about elided nodes in the final plan.

Store information about elided nodes in the final plan.

When setrefs.c removes a SubqueryScan, single-child Append, or
single-child MergeAppend from the final Plan tree, the RTI which
would have been scanned by the removed node no longer appears in
the final plan (the actual range table entry is still present,
but it's no longer referenced).

That's fine for the executor, but it can create difficulties for
code that wants to deduce from the final plan what choices were
made during the planing process. For example, a traversal of a
join tree in the final plan might never encounter the RTI of one
of the relationss in the join problem, and might instead encounter
a scan of a child RTI or even one from a different subquery level.

This patch adjusts things so that each time we elide a node during
setrefs processing, we record the plan_node_id of its single surviving
child, the type of the removed node, and the RTIs that the removed
node would have scanned. This information is recorded in a separate
list that can be ignored by the executor and examined only by code
that cares about these details.

This commit also updates pg_overexplain to display these details.