Summary:
- Backporting `std:exchange` is no longer needed as Folly requires C++14
  support and C++14-compliant vendors have `std::exchange` in their
  standard library, notably including GCC 4.9.
- Cut backport of `std::exchange` and fix call sites to explicitly use
  `std::exchange` where they were previously relying on `exchange` being
  found in the `folly` namespace.
Pull Request resolved: https://github.com/facebook/folly/pull/1021

Reviewed By: Orvid

Differential Revision: D14119875

Pulled By: yfeldblum

fbshipit-source-id: 686a8e812f6728281b432eae1586de28d21da9dd