Added get_tmp_table_item() to cache wrapper as it has all not simple Items (Item_func, Item_field, Item_subquery).

mysql-test/r/subquery_cache.result:
  Tests for bugs fixed.
mysql-test/t/subquery_cache.test:
  Tests for bugs fixed.
sql/item.cc:
  Added get_tmp_table_item() to cache wrapper as it has all not simple Items (Item_func, Item_field, Item_subquery).
sql/item.h:
  Added get_tmp_table_item() to cache wrapper as it has all not simple Items (Item_func, Item_field, Item_subquery).