[Clang] Emit a warning for ambiguous joined '-o' arguments

The offloading toolchain makes heavy use of options beginning with
`--o`. This is problematic when combined with the joined `-o` flag. In
the following situation, the user will not get the expected output and
will not notice as the expected output will still be written.
```
clang++ -x cuda foo.cu -offload-arch=sm_80 -o foo
```

This patch introduces a warning that checks for joined `-o` arguments
that would also be a valid driver argument if an additional `-` were
added. I believe this situation is uncommon enough to warrant a warning,
and can be trivially fixed by the end user by using the more common
separate form instead.

Reviewed By: tra, MaskRay

Differential Revision: https://reviews.llvm.org/D135389