Check the class of the copy made by Item::shallow_copy()

Item::shallow_copy() was meant to create a copy of the same class as the item it is called on, but nothing enforced that. A class that does not declare its own override inherits the one of its nearest concrete parent, and get_item_copy<Parent>() then silently creates an object of the parent class, losing the child's virtual behaviour (see MDEV-38801 for an earlier instance with Item_cache_year).