CUDA host device code with two code paths

Summary:
Allow CUDA host device functions with two code paths using __CUDA_ARCH__
to differentiate between code path being compiled.

For example:
  __host__ __device__ void host_device_function(void) {
  #ifdef __CUDA_ARCH__
    device_only_function();
  #else
    host_only_function();
  #endif
  }

Patch by Jacques Pienaar.

Reviewed By: rnk

Differential Revision: http://reviews.llvm.org/D6457

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@223271 91177308-0d34-0410-b5e6-96231b3b80d8