[Bug c++/123875] [13/14/15/16 Regression] atomic<X>(X) cosntructor is usable at compile time in C++14 and later

[tkaminsk at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123875

--- Comment #2 from Tomasz Kamiński <tkaminsk at gcc dot gnu.org> ---
We do not yet implement C++26 constexpr support for atomic, so we ignore
examples like for time being:
```
consteval void f()
{
  std::atomic<X> x(X{1, 2});
  X x1{};
  x.compare_exchange_strong(x1, X{3, 4});
}
```