Fix test/Driver/cl.c.

This test would fail in weird ways on systems with a one-letter filename
in the root directory, because the shell would helpfully expand /? to e.g. /n.

Make sure this doesn't happen by adding quotes.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187295 91177308-0d34-0410-b5e6-96231b3b80d8