Make ExtendedCoroutineHandle truthy when it holds a truthy basic handle for a non-extended promise

Reviewed By: simpkins

Differential Revision: D32838375

fbshipit-source-id: d8c350715d3a5fc7ebaa69f723cb6626bdc80031

folly/experimental/coro/Coroutine.h

@@ -220,7 +220,7 @@ class ExtendedCoroutineHandle {
     return {basic_, nullptr};
   }
 
-  explicit operator bool() const noexcept { return !!extended_; }
+  explicit operator bool() const noexcept { return !!basic_; }
 
  private:
   template <typename Promise>