Disambiguate the various in_place overloads, following C++17

Disambiguate the various in_place overloads, following C++17

Summary:
[Folly] Disambiguate the various `in_place` overloads, following C++17.

Forwarding `folly::in_place` is ambiguous because there are multiple overloads and we depend on deduction to choose the correct overload. For example:

```lang=c++
enum struct Err {};
Expected<Expected<int, Err>, Err> val(in_place, in_place, 3); // fails to compile
```

So we must disambiguate the three overloads: the default, the typed, and the indexed.

C++17 defines `std::in_place`, `std::in_place_type<typename>`, and `std::in_place_index<std::size_t>`. Let us mimic that exactly, so that it becomes trivial to swap out our implementations for the standard implementations once we jump to C++17.

Reviewed By: Orvid

Differential Revision: D5362339

fbshipit-source-id: d4012b01796390e74d8c14cdf68af70102608039