Pull in r155978 from upstream llvm trunk:

Fix unintentional use of operator bool.

This enables llvm's bugpoint tool to build with libc++.

MFC after:	3 days

contrib/llvm/tools/bugpoint/ToolRunner.cpp

@@ -128,7 +128,7 @@ static int RunProgramRemotelyWithTimeout(const sys::Path &RemoteClientPath,
       ErrorFile.close();
     }
 
-    errs() << OS;
+    errs() << OS.str();
   }
 
   return ReturnCode;