Summary:
[Folly] Hack around MSVC not supporting `constexpr` storage.

GCC and Clang do, but MSVC does not, support:

```lang=c++
extern int const num;
constexpr int const num = 3;
```

Work around it.

Reviewed By: elsteveogrande

Differential Revision: D6903413

fbshipit-source-id: 3a0957816d8d82718fb6187dc1803df1605f80aa