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

[jlebar]

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.

[jlebar]

Upon further consideration, I think I'm going to push this with post-submit review, as we know this test is quite broken, and I think this change is unlikely to make the situation worse.

test/Driver/output-file-cleanup.c

+// RUN: rm -f "%t.d" "%t1.s" "%t2.s" "%t3.s" "%t4.s" "%t5.s"
+//
 // RUN: touch %t.s
 // RUN: not %clang -S -DCRASH -o %t.s -MMD -MF %t.d %s
 // RUN: test ! -f %t.s
 // RUN: test ! -f %t.d
 
 // RUN: touch %t.s
 // RUN: not %clang -S -DMISSING -o %t.s -MMD -MF %t.d %s
 // RUN: test ! -f %t.s