libswiften: boost 1.56 compatibility

Resolve build failure against boost 1.56 discussed at #31428.

Patch at https://gist.github.com/tdsmith/278e6bdaa5502bc5a5f3 backported from upstream HEAD at http://swift.im/git/swift/commit/?id=381b22fc365c27b9cd585f4b78f53ebc698d9f54 and http://swift.im/git/swift/commit/?id=dc48cc3f34e3e229172202717520e77233c37ed7
Can you make this comment as explanatory as the PR/commit one? Thanks!