Implement LWG 2904 for std::variant assignment

Implement LWG 2904 for std::variant assignment

	* include/std/variant (__variant_construct): Use template parameter
	type instead of equivalent decltype-specifier.
	(_Move_ctor_base<false, Types...>::_Move_ctor_base(_Move_ctor_base&&)):
	Replace forward with move.
	(_Move_ctor_base<false, Types...>::_M_destructive_move)
	(_Move_ctor_base<false, Types...>::_M_destructive_copy)
	(_Move_ctor_base<true, Types...>::_M_destructive_move)
	(_Move_ctor_base<true, Types...>::_M_destructive_copy): Only set the
	index after construction succeeds.
	(_Copy_assign_base<false, Types...>::operator=): Remove redundant
	if-constexpr checks that are always true. Use __remove_cvref_t instead
	of remove_reference so that is_nothrow_move_constructible check
	doesn't use a const rvalue parameter. In the potentially-throwing case
	construct a temporary and move assign it, as per LWG 2904.
	(_Move_assign_base<false, Types...>::operator=): Remove redundant
	if-constexpr checks that are always true. Use emplace as per LWG 2904.
	(variant::operator=(T&&)): Only use emplace conditionally, otherwise
	construct a temporary and move assign from it, as per LWG 2904.
	* testsuite/20_util/variant/exception_safety.cc: Check that
	assignment operators have strong exception safety guarantee.

From-SVN: r270525