Add ALTER TYPE ... ADD/DROP/ALTER/RENAME ATTRIBUTE

Like with tables, this also requires allowing the existence of
composite types with zero attributes.

reviewed by KaiGai Kohei

src/include/nodes/parsenodes.h

@@ -1046,6 +1046,7 @@ typedef struct SetOperationStmt
 typedef enum ObjectType
 {
 	OBJECT_AGGREGATE,
+	OBJECT_ATTRIBUTE,			/* type's attribute, when distinct from column */
 	OBJECT_CAST,
 	OBJECT_COLUMN,
 	OBJECT_CONSTRAINT,