[Sema] attempt to appease buildbots after r367940

A buildbot got angry about this new test, with error messages like:

warn-nullchar-nullptr.c Line 16: use of undeclared identifier 'u'

It looks like this `u'c'` syntax was introduced in C11; I'm guessing
some bots may default to something before that. Let's see if explicitly
specifying the standard version makes it happy...

llvm-svn: 367947