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/commands/copyfrom.c

@@ -767,7 +767,7 @@ CopyFrom(CopyFromState cstate)
 	ExecInitResultRelation(estate, resultRelInfo, 1);
 
 	/* Verify the named relation is a valid target for INSERT */
-	CheckValidResultRel(resultRelInfo, CMD_INSERT);
+	CheckValidResultRel(resultRelInfo, CMD_INSERT, NIL);
 
 	ExecOpenIndices(resultRelInfo, false);