Fix up planner infrastructure to support LATERAL properly.

This patch takes care of a number of problems having to do with failure
to choose valid join orders and incorrect handling of lateral references
pulled up from subqueries.  Notable changes:

* Add a LateralJoinInfo data structure similar to SpecialJoinInfo, to
represent join ordering constraints created by lateral references.
(I first considered extending the SpecialJoinInfo structure, but the
semantics are different enough that a separate data structure seems
better.)  Extend join_is_legal() and related functions to prevent trying
to form unworkable joins, and to ensure that we will consider joins that
satisfy lateral references even if the joins would be clauseless.

* Fill in the infrastructure needed for the last few types of relation scan
paths to support parameterization.  We'd have wanted this eventually
anyway, but it is necessary now because a relation that gets pulled up out
of a UNION ALL subquery may acquire a reltargetlist containing lateral
references, meaning that its paths *have* to be parameterized whether or
not we have any code that can push join quals down into the scan.

* Compute data about lateral references early in query_planner(), and save
in RelOptInfo nodes, to avoid repetitive calculations later.

* Assorted corner-case bug fixes.

There's probably still some bugs left, but this is a lot closer to being
real than it was before.

src/backend/optimizer/prep/prepjointree.c

@@ -1026,11 +1026,14 @@ pull_up_simple_subquery(PlannerInfo *root, Node *jtnode, RangeTblEntry *rte,
 										subroot->append_rel_list);
 
 	/*
-	 * We don't have to do the equivalent bookkeeping for outer-join info,
-	 * because that hasn't been set up yet.  placeholder_list likewise.
+	 * We don't have to do the equivalent bookkeeping for outer-join or
+	 * LATERAL info, because that hasn't been set up yet.  placeholder_list
+	 * likewise.
 	 */
 	Assert(root->join_info_list == NIL);
 	Assert(subroot->join_info_list == NIL);
+	Assert(root->lateral_info_list == NIL);
+	Assert(subroot->lateral_info_list == NIL);
 	Assert(root->placeholder_list == NIL);
 	Assert(subroot->placeholder_list == NIL);
 
@@ -2319,6 +2322,7 @@ substitute_multiple_relids_walker(Node *node,
 	}
 	/* Shouldn't need to handle planner auxiliary nodes here */
 	Assert(!IsA(node, SpecialJoinInfo));
+	Assert(!IsA(node, LateralJoinInfo));
 	Assert(!IsA(node, AppendRelInfo));
 	Assert(!IsA(node, PlaceHolderInfo));
 	Assert(!IsA(node, MinMaxAggInfo));

src/backend/optimizer/prep/prepunion.c

@@ -1746,6 +1746,7 @@ adjust_appendrel_attrs_mutator(Node *node,
 	}
 	/* Shouldn't need to handle planner auxiliary nodes here */
 	Assert(!IsA(node, SpecialJoinInfo));
+	Assert(!IsA(node, LateralJoinInfo));
 	Assert(!IsA(node, AppendRelInfo));
 	Assert(!IsA(node, PlaceHolderInfo));
 	Assert(!IsA(node, MinMaxAggInfo));