Rearrange "add column" logic to merge columns at exec time.

The previous coding set attinhcount too high in some cases, resulting in
an undumpable, undroppable column.  Per bug #5856, reported by Naoya
Anzai.  See also commit 31b6fc06d8, which
fixes a similar bug in ALTER TABLE .. ADD CONSTRAINT.

Patch by Noah Misch.

src/include/nodes/parsenodes.h

@@ -1173,6 +1173,7 @@ typedef struct AlterTableStmt
 typedef enum AlterTableType
 {
 	AT_AddColumn,				/* add column */
+	AT_AddColumnRecurse,		/* internal to commands/tablecmds.c */
 	AT_AddColumnToView,			/* implicitly via CREATE OR REPLACE VIEW */
 	AT_ColumnDefault,			/* alter column default */
 	AT_DropNotNull,				/* alter column drop not null */
@@ -1198,6 +1199,7 @@ typedef enum AlterTableType
 	AT_ClusterOn,				/* CLUSTER ON */
 	AT_DropCluster,				/* SET WITHOUT CLUSTER */
 	AT_AddOids,					/* SET WITH OIDS */
+	AT_AddOidsRecurse,			/* internal to commands/tablecmds.c */
 	AT_DropOids,				/* SET WITHOUT OIDS */
 	AT_SetTableSpace,			/* SET TABLESPACE */
 	AT_SetRelOptions,			/* SET (...) -- AM specific parameters */