cpp/experimental/barrier/arrive and wait

Blocks and arrive at the barrier's synchronization point.

The behavior is undefined if the calling thread is not in the set of participating threads of this barrier.

Calls to synchronizes with the start of the completion phase of the barrier. The completion of the completion phase synchronizes with the return from the call.

Calls to and  never introduce data races with themselves or each other.

Exceptions
Throws nothing.