Based on the current logic, objects of classes Item_func_charset and
Item_func_coercibility (responsible for CHARSET() and COERCIBILITY()
functions) are always considered constant.
However, SQL syntax allows their use in a non-constant manner, such as
CHARSET(t1.a), COERCIBILITY(t1.a).

In these cases, the `used_tables()` parameter corresponds to table names
in the function parameters, creating an inconsistency: the item is marked
as constant but accesses tables. This leads to crashes when
conditions with CHARSET()/COERCIBILITY() are pushed into derived tables.

This commit addresses the issue by setting `used_tables()` to 0 for
`Item_func_charset` and `Item_func_coercibility`. Additionally, the items
now store the return values during the preparation phase and return
them during the execution phase. This ensures that the items do not call
its arguments methods during the execution and are truly constant.

Reviewer: Alexander Barkov <bar@mariadb.com>