MDEV-4265 5.5 is slower than 5.3 because of many str_to_datetime calls

get_datetime_value() should not double-cache its own Item_cache_temporal items,
but it *should* cache other Item_cache items, such as Item_cache_str.

sql/item.h:
  shortcut, to avoid going through the switch in Item::cmp_type()
sql/item_cmpfunc.cc:
  even if the item is Item_cache_str - it still needs to be converted and cached.
sql/item_timefunc.h:
  all descendants of Item_temporal_func always have cmp_type==TIME_RESULT.
  Even Item_date_add_interval, that might have field_type == MYSQL_TYPE_STRING.

sql/sql_time.cc

@@ -302,6 +302,9 @@ str_to_datetime_with_warn(CHARSET_INFO *cs,
     make_truncated_value_warning(thd, MYSQL_ERROR::WARN_LEVEL_WARN,
                                  str, length, flags & TIME_TIME_ONLY ?
                                  MYSQL_TIMESTAMP_TIME : ts_type, NullS);
+  DBUG_EXECUTE_IF("str_to_datetime_warn",
+                  push_warning(thd, MYSQL_ERROR::WARN_LEVEL_NOTE,
+                               ER_YES, str););
   return ts_type;
 }