[Sema] Attempt to fix tests for utf-8 invalid format string specifiers

Followup from r264752.

Attempt to appease buildbots:
 http://lab.llvm.org:8011/builders/clang-ppc64be-linux/builds/2882
 http://lab.llvm.org:8011/builders/clang-s390x-linux/builds/2619

llvm-svn: 264765