Remove several unused variables.

Patch by Alp Toker.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191757 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/GraphWriter.cpp

@@ -219,5 +219,8 @@ void llvm::DisplayGraph(StringRef FilenameRef, bool wait,
   errs() << "Running 'dotty' program... ";
   if (!ExecGraphViewer(dotty, args, Filename, wait, ErrMsg))
     return;
+#else
+  (void)Filename;
+  (void)ErrMsg;
 #endif
 }