partially revert 76063c2a. Item::clone() is not an all-purpose
Item copying machine, it was specifically created for pushdown
of predicates into derived tables and views and it does not
copy everything. In particular, it does not copy Item_func_regex.

Fix the bug differently by preserving the old constraint name.
But keep setting automatic_name=true to have it regenerated
for cases like ALTER TABLE ... ADD CONSTRAINT.