Don't actually emit output in test/Headers/mmprefetch.c.

The test directory cannot assumed to be writable.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201768 91177308-0d34-0410-b5e6-96231b3b80d8

test/Headers/mmprefetch.c

-// RUN: %clang_cc1 -triple=x86_64-unknown-unknown -fms-extensions -emit-llvm -verify %s
+// RUN: %clang_cc1 -triple=x86_64-unknown-unknown -fms-extensions -emit-llvm-only -verify %s
 
 #include <mmintrin.h>