Adapt to LLVM optimization remark interface change. NFC

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@282540 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/CodeGenAction.cpp

@@ -514,7 +514,7 @@ void BackendConsumer::EmitOptimizationMessage(
 
   std::string Msg;
   raw_string_ostream MsgStream(Msg);
-  MsgStream << D.getMsg().str();
+  MsgStream << D.getMsg();
 
   if (D.getHotness())
     MsgStream << " (hotness: " << *D.getHotness() << ")";