Add a module cache path to test/Headers/cxx11

Don't depend on the serialization format remaining the same.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215454 91177308-0d34-0410-b5e6-96231b3b80d8

test/Headers/cxx11.cpp

+// RUN: rm -rf %t
 // RUN: %clang_cc1 -ffreestanding -fsyntax-only -std=c++11 %s
-// RUN: %clang_cc1 -ffreestanding -fsyntax-only -std=c++11 -fmodules %s
+// RUN: %clang_cc1 -ffreestanding -fsyntax-only -std=c++11 -fmodules -fmodules-cache-path=%t %s
 
 // This test fails on systems with older OS X 10.9 SDK headers, see PR18322.