Make having no RUN line a failure.

Doug, please look at decltype-crash and instantiate-function-1.mm, I'm not sure
if they are actually testing the right thing / anything.

llvm-svn: 77070
diff --git a/clang/test/Sema/attr-format_arg.c b/clang/test/Sema/attr-format_arg.c
index 6f95315..0830951 100644
--- a/clang/test/Sema/attr-format_arg.c
+++ b/clang/test/Sema/attr-format_arg.c
@@ -1,3 +1,5 @@
+// RUN: clang-cc -fsyntax-only -verify %s
+
 #include <stdio.h>
 
 const char* f(const char *s) __attribute__((format_arg(1)));