Don't feed standard error to FileCheck.

This is an attempt to fix the buildbot that started failing after
r270808.

http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-fast/builds/13141

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@270817 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit c642dab5e7f1cb2fe23bb12c8bdab2a5d5b7de5a)
1 file changed