Improve ALTER DOMAIN / DROP CONSTRAINT with nonexistent constraint

ALTER DOMAIN / DROP CONSTRAINT on a nonexistent constraint name did
not report any error.  Now it reports an error.  The IF EXISTS option
was added to get the usual behavior of ignoring nonexistent objects to
drop.

src/include/nodes/parsenodes.h

@@ -1264,6 +1264,7 @@ typedef struct AlterDomainStmt
 	char	   *name;			/* column or constraint name to act on */
 	Node	   *def;			/* definition of default or constraint */
 	DropBehavior behavior;		/* RESTRICT or CASCADE for DROP cases */
+	bool		missing_ok;		/* skip error if missing? */
 } AlterDomainStmt;