Make test/Driver/clang_f_opts.c not write to the test dir

After r195009, the test would write a .o file to the test dir. Send that to
/dev/null instead. Also fix the typo in test/Frontend/invalid-o-level.c.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195047 91177308-0d34-0410-b5e6-96231b3b80d8