Update folly's source of truth to xplat
Update folly's source of truth to xplat

Reviewed By: mzlee

Differential Revision: D23066827

fbshipit-source-id: 6362acb7a6680a24871859d7874a9caec0a1538e

ReactiveSocket/reactivesocket-cpp,ReactiveSocket/reactivesocket-cpp,rsocket/rsocket-cpp,rsocket/rsocket-cpp,rsocket/rsocket-cpp,rsocket/rsocket-cpp,ReactiveSocket/reactivesocket-cpp