[Driver] Don't assert when some files /don't/ get rebuilt but we still merge modules.

...or do anything else that depends on the compile commands that get skipped.

Fix-up to rdar://problem/21129029

Swift SVN r29816

include/swift/Basic/TaskQueue.h

@@ -136,6 +136,12 @@ public:
   execute(TaskBeganCallback Began = TaskBeganCallback(),
           TaskFinishedCallback Finished = TaskFinishedCallback(),
           TaskSignalledCallback Signalled = TaskSignalledCallback());
+
+  /// Returns true if there are any tasks that have been queued but have not
+  /// yet been executed.
+  bool hasRemainingTasks() {
+    return !QueuedTasks.empty();
+  }
 };
 
 /// \brief A class which simulates execution of tasks with behavior similar to