New runtime type diagnostic (MDEV-34490) has detected that classes
Item_func_eq, Item_default_value and Item_date_literal_for_invalid_dates
incorrectly return an instance of its ancestor classes when being cloned.
This commit fixes that.

Additionally, it fixes a bug at Item_func_case_simple::do_build_clone()
which led to an endless loop of cloning functions calls.

Reviewer: Oleksandr Byelkin <sanja@mariadb.com>