Fix PlaceHolderVar mechanism's interaction with outer joins.

The point of a PlaceHolderVar is to allow a non-strict expression to be
evaluated below an outer join, after which its value bubbles up like a Var
and can be forced to NULL when the outer join's semantics require that.
However, there was a serious design oversight in that, namely that we
didn't ensure that there was actually a correct place in the plan tree
to evaluate the placeholder :-(.  It may be necessary to delay evaluation
of an outer join to ensure that a placeholder that should be evaluated
below the join can be evaluated there.  Per recent bug report from Kirill
Simonov.

Back-patch to 8.4 where the PlaceHolderVar mechanism was introduced.

src/backend/nodes/outfuncs.c

@@ -1768,6 +1768,7 @@ _outPlaceHolderInfo(StringInfo str, PlaceHolderInfo *node)
 	WRITE_NODE_FIELD(ph_var);
 	WRITE_BITMAPSET_FIELD(ph_eval_at);
 	WRITE_BITMAPSET_FIELD(ph_needed);
+	WRITE_BITMAPSET_FIELD(ph_may_need);
 	WRITE_INT_FIELD(ph_width);
 }