explicitly declare virtual destructors

Author: ktoso

stdlib/public/Concurrency/TaskGroup.cpp

@@ -666,6 +666,8 @@ class AccumulatingTaskGroup: public TaskGroupBase {
 
   virtual void destroy() override;
 
+  virtual ~AccumulatingTaskGroup() {}
+
   virtual bool isDiscardingResults() const override {
     return false;
   }
@@ -714,6 +716,8 @@ class DiscardingTaskGroup: public TaskGroupBase {
 
   virtual void destroy() override;
 
+  virtual ~DiscardingTaskGroup() {}
+
   virtual bool isDiscardingResults() const override {
     return true;
   }