PR libstdc++/85184 remove debug assertions from std::variant

PR libstdc++/85184 remove debug assertions from std::variant

The __glibcxx_assert macro should be used to check preconditions that
users must meet, not to check postconditions that the implementation
must meet. We have tests to verify std::variant meets its
postconditions, users shouldn't pay for those checks at runtime.

	PR libstdc++/85184
	* include/std/variant (_Copy_assign_base, _Move_assign_base, variant):
	Remove assertions.
	(variant::emplace<_Tp>): Remove result of emplace<N> directly.

From-SVN: r270117