MDEV-16852 Get rid of Item_temporal_hybrid_func::fix_temporal_type()

- Implementing the task according to the MDEV description.
- Adding a helper class Sec6_add to share the code in type-specific
  branches in Item_func_add_time::get_date().

sql/item_timefunc.h

@@ -620,11 +620,6 @@ public:
   my_decimal *val_decimal(my_decimal *decimal_value)
   { return  val_decimal_from_date(decimal_value); }
 
-  /**
-    Fix the returned timestamp to match field_type(),
-    which is important for val_str().
-  */
-  bool fix_temporal_type(MYSQL_TIME *ltime);
   /**
     Return string value in ASCII character set.
   */