Revert "Using an invalid -O falls back on -O3 instead of an error"

Trying to fix test failures since earlier today.

One of the tests added in this commit is outputting test/Driver/clang_f_opts.s
which the builders that build in-tree (eg. clang-native-arm-cortex-a9) are
trying to run as a test case, causing failures.

clang_f_opts.c:
  If -### doesn't emit the warning then this test probably shouldn't be in
  here in the first place. Frontend maybe?

invalid-o-level.c:
  Running %clang_cc1 in the Driver tests doesn't make sense because -cc1
  bypasses the driver. (I'm not reverting the commit that introduced this but
  please fix instead of keeping it this way.)

Reverting to fix the build failures and also so that the tests can be thought
out more thoroughly.

This reverts commit r194817.