Silence -Wreturn-type warning

llvm-svn: 210005
diff --git a/llvm/lib/Support/GraphWriter.cpp b/llvm/lib/Support/GraphWriter.cpp
index c26f6a1..1521475 100644
--- a/llvm/lib/Support/GraphWriter.cpp
+++ b/llvm/lib/Support/GraphWriter.cpp
@@ -127,6 +127,7 @@
   case GraphProgram::CIRCO:
     return "circo";
   }
+  llvm_unreachable("bad kind");
 }
 
 bool llvm::DisplayGraph(StringRef FilenameRef, bool wait,