add a comment to explain this test case

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@161172 91177308-0d34-0410-b5e6-96231b3b80d8

test/CodeGen/nobuiltin.c

@@ -1,7 +1,8 @@
 // RUN: %clang_cc1 -fno-builtin -O1 -S -o - %s | FileCheck %s
 
-void fn() {
+void PR13497() {
   char content[2];
+  // make sure we don't optimize this call to strcpy()
   // CHECK: __strcpy_chk
   __builtin___strcpy_chk(content, "", 1);
 }