Fix nested PlaceHolderVar expressions that appear only in targetlists.

A PlaceHolderVar's expression might contain another, lower-level
PlaceHolderVar.  If the outer PlaceHolderVar is used, the inner one
certainly will be also, and so we have to make sure that both of them get
into the placeholder_list with correct ph_may_need values during the
initial pre-scan of the query (before deconstruct_jointree starts).
We did this correctly for PlaceHolderVars appearing in the query quals,
but overlooked the issue for those appearing in the top-level targetlist;
with the result that nested placeholders referenced only in the targetlist
did not work correctly, as illustrated in bug #6154.

While at it, add some error checking to find_placeholder_info to ensure
that we don't try to create new placeholders after it's too late to do so;
they have to all be created before deconstruct_jointree starts.

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

src/backend/optimizer/path/equivclass.c

@@ -850,7 +850,7 @@ generate_base_implied_equalities_no_const(PlannerInfo *root,
 										   PVC_RECURSE_AGGREGATES,
 										   PVC_INCLUDE_PLACEHOLDERS);
 
-		add_vars_to_targetlist(root, vars, ec->ec_relids);
+		add_vars_to_targetlist(root, vars, ec->ec_relids, false);
 		list_free(vars);
 	}
 }