Fix the parameters order for TableAmRoutine.relation_copy_for_cluster()

Specify OldTable first, NewTable second as used by
table_relation_copy_for_cluster() and as implemented in
heapam_relation_copy_for_cluster().

Backpatch to PostgreSQL 12, where TableAmRoutine was introduced.

Discussion: https://postgr.es/m/ME3P282MB3166860D4911AE82F92DF7C5B63F2%40ME3P282MB3166.AUSP282.PROD.OUTLOOK.COM
Author: Japin Li
Reviewed-by: Pavel Borisov
Backpatch-through: 12

src/include/access/tableam.h

@@ -601,8 +601,8 @@ typedef struct TableAmRoutine
 									   const RelFileNode *newrnode);
 
 	/* See table_relation_copy_for_cluster() */
-	void		(*relation_copy_for_cluster) (Relation NewTable,
-											  Relation OldTable,
+	void		(*relation_copy_for_cluster) (Relation OldTable,
+											  Relation NewTable,
 											  Relation OldIndex,
 											  bool use_sort,
 											  TransactionId OldestXmin,