Fixed two problems VC++ revealed in release mode. Please verify.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@41995 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/AST/CFG.cpp b/AST/CFG.cpp
index 268fc06..1f08e60 100644
--- a/AST/CFG.cpp
+++ b/AST/CFG.cpp
@@ -1292,6 +1292,7 @@
 struct DOTGraphTraits<const CFG*> : public DefaultDOTGraphTraits {
   static std::string getNodeLabel(const CFGBlock* Node, const CFG* Graph) {
 
+#ifndef NDEBUG
     std::ostringstream Out;
     print_block(Out,Graph, *Node, GraphHelper, false);
     std::string OutStr = Out.str();
@@ -1306,6 +1307,9 @@
       }
       
     return OutStr;
+#else
+    return "";
+#endif
   }
 };
 } // end namespace llvm