[CUDA] Handle -O options (more) correctly.

Summary:
Previously we'd crash the driver if you passed -O0.  Now we try to
handle all of clang's various optimization flags in a sane way.

Reviewers: tra

Subscribers: cfe-commits, echristo, jhen

Differential Revision: http://reviews.llvm.org/D16307

llvm-svn: 258174