MDEV-324: SHOW EXPLAIN: Plan produced by SHOW EXPLAIN for a query with TEMPTABLE view loses 'DERIVED' line

- Make SHOW EXPLAIN code take into account that st_select_lex object without joins can be
  a full-featured SELECTs which were already executed and cleaned up.

sql/sql_class.h

@@ -3328,6 +3328,8 @@ public:
   /* this will be called in the parent thread: */
   void flush_data();
 
+  void discard_data();
+
   List<String> data_rows;
 };