US355 Make atomic_ref<T>::notify_one/all const

[wg21bot]

31.07 [atomics.ref.generic]
31.7.1 [atomics.ref.operations]
31.7.2 [atomics.ref.int]
31.7.3 [atomics.ref.float]
31.7.4 [atomics.ref.pointer]

atomic_ref<T>::notify_one and atomic_ref<T>::notify_all should be const member functions (in the generic class and all the specializations), since it is the atomic object that is (conceptually) modified, not the atomic_ref<T> object.

Proposed change:
Make atomic_ref<T>::notify_one and atomic_ref<T>::notify_all const.

[ogiroux]

Recommend to make notify_one, notify_all, and operator=(float) of atomic_ref const,
in response to US 358 and US 355:
SF F N A SA
Unanimous consent

[tituswinters]

LEWG in Belfast: We agree with SG1, forward to LWG for C++20.

[jensmaurer]

Accepted.