Fix futures MSVC compilation issue
Fix futures MSVC compilation issue

Summary:
MSVC (14.12.25827, VS 2017 15.5) fails to compile with internal error when using `/std:c++14` parameter:
```
Future-inl.h(1484): fatal error C1001: An internal error has occurred in the compiler.
(compiler file 'msc1.cpp', line 1507)
 To work around this problem, try simplifying or changing the program near the locations listed above.
```

Reviewed By: yfeldblum, Orvid

Differential Revision: D10018915

fbshipit-source-id: ba886265b68684aacc2bf471c79ba6c39e611c8e

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly