Implement correct std::variant triviality rules from P0602R4

Implement correct std::variant triviality rules from P0602R4

The std::variant move assignment operator should not be trivial if the
variant is not trivially move constructible.

	* include/std/variant (__detail::__variant::_Traits): Make
	_S_trivial_copy_assign depend on _S_trivial_copy_ctor and make
	_S_trivial_move_assign depend on _S_trivial_move_ctor, as per
	P0602R4.
	(__detail::__variant::_Copy_assign_alias): Only depend on
	_S_trivial_copy_assign, which subsumes _S_trivial_copy_ctor now.
	* testsuite/20_util/variant/compile.cc: Correct checks for trivial
	move assignment operators.

From-SVN: r270510