<atomic>: Add missing atomic& operator=(const atomic&) volatile = delete; overload

[frederick-vs-ja]

Per [atomics.types.generic.general], [atomics.types.int], [atomics.types.float], and [atomics.types.pointer], this overload exists for all specializations.

Unblocks two libcxx tests:

• std/atomics/atomics.types.generic/atomics.types.float/copy.compile.pass.cpp
• std/atomics/atomics.types.generic/copy_semantics_traits.pass.cpp

[StephanTLavavej]

I verified that there are two unaffected partial specializations (in the Standard and our implementation): atomic<shared_ptr<T>> and atomic<weak_ptr<T>>.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for investigating these libcxx failures and improving atomic conformance! 😻 ☢️ ✅