Silence compiler warning.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@144386 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/tools/c-index-test/c-index-test.c b/tools/c-index-test/c-index-test.c
index 0860f18..833c5b4 100644
--- a/tools/c-index-test/c-index-test.c
+++ b/tools/c-index-test/c-index-test.c
@@ -1617,6 +1617,7 @@
   case CXIdxEntity_Union: return "union";
   case CXIdxEntity_CXXClass: return "c++-class";
   }
+  return "<unknown>";
 }
 
 static void printEntityInfo(const char *cb,