<functional>: unwrap move_only_function with abominable and noexcept types

[AlexGuteniev]

Towards #5504.
Follow up on #5808.

📦 Newly unwrapped cases

• move_only_function<R(Args...) cv1-ref1-noex1>(move_only_function<R(Args...) cv2-ref2-noex2>(callable)); where noexcept and const may be present in only inner, and & or && may be present in only outer
• move_only_function<R(Args...) cv-ref>(function<R(Args...)>(callable)) where cv-ref can have const, &, and &&.

In the tests make sure the type system is not broken, and the cases which are not supposed to compile, still do not compile.

🐿️ Squirrelly case

Inner noexcept, outer non-noexcept case is squirrelly.

If we keep conditional noexcept on the structure, it would violate strict aliasing: the inner one would have a different _Function_base and thus _Function_base::_Impl_t.

Alternatives:

• To fix that, we remove _Noexcept parameter from _Function_base, and pass that as template parameter on function construction, also dress it onto _Invoke pointer with reinterpret_cast when needed. This is doable, but makes the current change more complicated.
• Not have noexcept varying case unwrapped.
• Not use noexcept function pointer (bad, missed optimization we're asked to do).
• Have completely untyped _Impl_t (need to get that working in clang-cl somehow, as it can't assign function pointer to void*)