PR libstdc++/86874 fix std::variant::swap regression

PR libstdc++/86874 fix std::variant::swap regression

	PR libstdc++/86874
	* include/std/variant (_Copy_ctor_base::_M_destructive_move): Define
	here instead of in _Move_assign_base.
	(_Copy_ctor_base<true, _Types...>::_M_destructive_move): Define.
	(_Copy_assign_base::operator=): Use _M_destructive_move when changing
	the contained value to another alternative.
	(_Move_assign_base::operator=): Likewise.
	(_Move_assign_base::_M_destructive_move): Remove.
	* testsuite/20_util/variant/86874.cc: New test.

From-SVN: r263365