Summary:
A semaphore type which wraps the platform native semaphore type.

On Windows, this is the Windows semaphore type. On Apple, this is the semaphore type from libdispatch. Otherwise, this is the POSIX semaphore type, which is deprecated on Apple and unavailable on Windows.

Reviewed By: Orvid

Differential Revision: D27951114

fbshipit-source-id: e6a870f656eecd3ba41c2efed038b916c124d0e8