[clang] Make Driver tests pass when running with temp dir containing "crt"

In a recent run, temp files got created in /tmp/lit-tmp-2wcrtcx1/foo-xxx.o.
Since the tmp path contained "crt", this made a few tests fail:
http://45.33.8.238/linux/72221/step_7.txt

Not allowing '/' as path of the file name prevents this.

Differential Revision: https://reviews.llvm.org/D122547