Support MERGE into updatable views.

This allows the target relation of MERGE to be an auto-updatable or
trigger-updatable view, and includes support for WITH CHECK OPTION,
security barrier views, and security invoker views.

A trigger-updatable view must have INSTEAD OF triggers for every type
of action (INSERT, UPDATE, and DELETE) mentioned in the MERGE command.
An auto-updatable view must not have any INSTEAD OF triggers. Mixing
auto-update and trigger-update actions (i.e., having a partial set of
INSTEAD OF triggers) is not supported.

Rule-updatable views are also not supported, since there is no
rewriter support for non-SELECT rules with MERGE operations.

Dean Rasheed, reviewed by Jian He and Alvaro Herrera.

Discussion: https://postgr.es/m/CAEZATCVcB1g0nmxuEc-A+gGB0HnfcGQNGYH7gS=7rq0u0zOBXA@mail.gmail.com

src/backend/executor/execPartition.c

@@ -361,7 +361,7 @@ ExecFindPartition(ModifyTableState *mtstate,
 				if (rri)
 				{
 					/* Verify this ResultRelInfo allows INSERTs */
-					CheckValidResultRel(rri, CMD_INSERT);
+					CheckValidResultRel(rri, CMD_INSERT, NIL);
 
 					/*
 					 * Initialize information needed to insert this and
@@ -527,7 +527,7 @@ ExecInitPartitionInfo(ModifyTableState *mtstate, EState *estate,
 	 * partition-key becomes a DELETE+INSERT operation, so this check is still
 	 * required when the operation is CMD_UPDATE.
 	 */
-	CheckValidResultRel(leaf_part_rri, CMD_INSERT);
+	CheckValidResultRel(leaf_part_rri, CMD_INSERT, NIL);
 
 	/*
 	 * Open partition indices.  The user may have asked to check for conflicts