[CUDA] Make all host-side shadows of device-side variables undef.

The host-side code can't (and should not) access the values that may
only exist on the device side. E.g. address of a __device__ function
does not exist on the host side as we don't generate the code for it there.

Differential Revision: https://reviews.llvm.org/D55663

llvm-svn: 349087