Mark LWG Issue #2288 as complete. This was wording cleanup, no code changes required.
Also mark #2104 as complete. Leave the implementation in libc++ as noexcept, since
implementations are allowed to add noexcept to non-virtual calls. If we throw from
unique_lock& operator=(unique_lock&& u), then that means the preconditions were violated,
and calling terminate() (as a result of throwing from a noexcept function) is as
good example of undefined behavior as any other.