* Item_default_value::fix_fields creates a copy of its argument's field.
* Field::default_value is changed when its expression is prepared in
  unpack_vcol_info_from_frm()

This means we must unpack any vcol expression that includes DEFAULT(x)
strictly after unpacking x->default_value.

To avoid building and solving this dependency graph on every table open,
we update Item_default_value::field->default_value after all vcols
are unpacked and fixed.