Make test/Driver/output-file-cleanup.c hermetic.

Summary:
It checks that certain files do and exist, so make sure that they don't
exist at the beginning of the test.

This hid a failure in r260448; to see the failure, you had to run the test with
a clean-ish objdir.

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D17216

llvm-svn: 261773