[CUDA] Add --cuda-compile-host-device, which overrides --cuda-host-only and --cuda-device-only.

Summary:
This completes the flag's tristate, letting you override it at will on
the command line.

Reviewers: tra

Subscribers: cfe-commits, jhen

Differential Revision: http://reviews.llvm.org/D19248

llvm-svn: 266707