Subzero: Manage each Cfg as a std::unique_ptr<Cfg>.

src/IceGlobalContext.h

Index: src/IceGlobalContext.h
diff --git a/src/IceGlobalContext.h b/src/IceGlobalContext.h
index 79c23d61c7e38e7d33eba5c6df6d0570de59dd3b..5d4e1c339d539d9f2ded1fb13720b65195b08546 100644
--- a/src/IceGlobalContext.h
+++ b/src/IceGlobalContext.h
@@ -282,7 +282,7 @@ public:
   // the work queue is currently empty.  Returns nullptr if there is
   // no more work - the queue is empty and either end() has been
   // called or the Sequential flag was set.
-  Cfg *cfgQueueBlockingPop() { return CfgQ.blockingPop(); }
+  std::unique_ptr<Cfg> cfgQueueBlockingPop();
   // Notifies that no more work will be added to the work queue.
   void cfgQueueNotifyEnd() { CfgQ.notifyEnd(); }