[PATCH] CUDA host device code with two code paths

[Reid Kleckner]

>>! In D6457#7, @pcc wrote:
> There is already a flag for this, `-fcuda-is-device`. We should make that flag a `LangOption` and use it for this check.

Right, this seems like the correct approach. Is there concern that it would be too disruptive at this stage to define `__CUDA_ARCH__`?

http://reviews.llvm.org/D6457