[libc++] Fix bug in tests for std::atomic_ref<T*> increment and decrement operators (#122271)

The implementation is fine and has the proper increment/decrement
operators defined, but the tests were wrong:
- a typo (`T` instead of `std::atomic_ref<T>`) when ensuring that increment/decrement
  operators are not defined in the primary template and specialization for floating point
  types, and
- the specialization for pointer types was miscategorized.

GitOrigin-RevId: 19557a4c8fab0dbfe9d9c53b99b7960ef211684e
1 file changed