A well-known instance of folly::Unit

Summary: [Folly] A well-known instance of `folly::Unit`.

Reviewed By: jsedgwick

Differential Revision: D2693426

fb-gh-sync-id: fe595abc852500275a827b4f400cfda7091a55f7

folly/futures/Unit.h

@@ -62,4 +62,6 @@ template <class T>
 struct is_void_or_unit : public Unit::Lift<T>
 {};
 
+constexpr Unit unit {};
+
 }