Item clones (deep copies) validation

During fixing MDEV-36686 a more general issue was observed: `do_build_clone()` functions of class `Item` descendants are supposed to produce deep copies of instances but often produce shallow copies. For example, `Item_field`:

Item* Item_field::do_build_clone(THD *thd) const override { return get_copy(thd); }

`get_copy()` performs shallow copying of an object.

Also, `do_build_clone()` implementation is sometimes omitted at all and is inherited from the parent class. In most cases it is not a problem: clones and shallow copies are equivalent. However, for some `Item` descendants it is not true, like for `Item_default_value`: it has

Item *arg

member that has to be deep copied, not shallow copied.

It's easy to overlook the presence of such members in a class, inheritance may also make such things less visible. So, the idea is to add a check whether a deep copy is indeed deep and not shallow.

So far the easiest way seems to be a debug runtime walk()-based sub-item collection, something like:

bool Item::collect_subitems_processor(void *arg)

{

  HashSet<Item*> *items = (HashSet<Item*>*) arg;

  items->insert(this);

  return false;

}

// In build_clone():

HashSet<Item*> original_items, clone_items;

this->walk(&Item::collect_subitems_processor, false, &original_items);

clone->walk(&Item::collect_subitems_processor, false, &clone_items);

// Check intersection (excluding clone itself)