Summary:
- Instead of defining our own version of whether a type is nothrow
  swappable, reuse the existing definition from `folly/Traits.h` which
  is spelled `IsNothrowSwappable`.
Pull Request resolved: https://github.com/facebook/folly/pull/1049

Reviewed By: Orvid

Differential Revision: D14402098

Pulled By: yfeldblum

fbshipit-source-id: 5b61bd6d43db939b4499e908b26a3eb61f8929a3