This test case was missing -fsyntax-only, so I've added it. NFC to the actual...

This test case was missing -fsyntax-only, so I've added it. NFC to the actual test contents, just how the test is executed.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@316028 91177308-0d34-0410-b5e6-96231b3b80d8

test/CXX/dcl.dcl/dcl.attr/dcl.attr.fallthrough/p1.cpp

-// RUN: %clang_cc1 -std=c++1z -verify %s
+// RUN: %clang_cc1 -fsyntax-only -std=c++1z -verify %s
 
 void f(int n) {
   switch (n) {