`std::in_place_type_t` constructor is missing from the paper

[Twon]

@jbcoe I assume this is an oversight on our part? Let me know and I can address this.

[jbcoe]

@Twon pure oversight. Please go ahead.

[Twon]

Also looks like we have missed of the std::allocator_arg_t constructor (but it is on the reference implementation).

  template <class U, class A, class = std::enable_if_t<std::is_convertible_v<U*, T*>>>
  ISOCPP_P0201_CONSTEXPR_CXX20 constexpr polymorphic_value(U* u, std::allocator_arg_t, const A& alloc);