This freezes the clone even if the receiver is not frozen. It
is only for consistency with freeze: false not freezing the clone
even if the receiver is frozen.
Because Object#clone is now partially implemented in Ruby and
not fully implemented in C, freeze: nil must be supported to
provide the default behavior of only freezing the clone if the
receiver is frozen.
This requires modifying delegate and set, to set freeze: nil
instead of freeze: true as the keyword parameter for
initialize_clone. Those are the two libraries in stdlib that
override initialize_clone.
Implements [Feature #16175]
