P3936R1: Safer `atomic_ref::address` (FR-030-310)

[frederick-vs-ja]

Link: https://wg21.link/P3936R1

Feature-test macro (updated):

#define __cpp_lib_atomic_ref                        202603L // freestanding, also in <atomic>

Feature-test macro update was not in the paper, but decided at straw poll (per cplusplus/draft issue 8842).