[driver] Parse diagnostic args in the driver.

Previously, only diagnostics thrown by the cc1 process were
actually honoring the diagnostic options given on the command line,
like -Werror.

Reuse the existing code in Frontend currently used for cc1,
adjusting it to not interpret -Wl, linker flags as warnings.

Also fix a faulty test exposed by this change.
It wasn't actually testing anything, and was giving this warning:

clang-3: warning: argument unused during compilation: '-verify'

Which -Werror didn't turn into an error because it was output
by the driver, not the cc1 process, and diagnostic options
weren't parsed by the driver. And you couldn't see the warning
when running the test suite.

Fixes PR12181.
Patch by Dylan Noblesmith <nobled@dreamwidth.org>.

llvm-svn: 152660