This allows removing all the code in CTranslatorDXLToPlStmt that tracked
the parent of each call.

I found the plan node IDs awkward, when I was hacking on
CTranslatorDXLToPlStmt. I tried to make a change where a function would
construct a child Plan node first, and a Result node on top of that, but
only if necessary, depending on the kind of child plan. The parent plan
node IDs made it impossible to construct a part of Plan tree like that, in
a bottom-up fashion, because you always had to pass the parent's ID when
constructing a child node. Now that is possible.