Support compiling in C++14 mode
Support compiling in C++14 mode

Summary:
The rest of Folly isn't dependent on C++17 mode in MSVC, so modify this to support not having it enabled.
This will make adoption easier as C++17 mode acts on the things the standard says are removed.

Reviewed By: yfeldblum

Differential Revision: D4509919

fbshipit-source-id: f4644f5f5ba78de8ab5192b89ac6320767b51084

Orvid/folly,rklabs/folly,facebook/folly,floxard/folly,rklabs/folly,facebook/folly,facebook/folly,rklabs/folly,Orvid/folly,Orvid/folly,facebook/folly,floxard/folly,rklabs/folly,Orvid/folly,facebook/folly,rklabs/folly,floxard/folly,floxard/folly,Orvid/folly,floxard/folly