[CUDA][HIP] Allow CUDA __global__ functions to have amdgpu kernel attributes

There are HIP applications e.g. Tensorflow 1.3 using amdgpu kernel attributes, however
currently they are only allowed on OpenCL kernel functions.

This patch will allow amdgpu kernel attributes to be applied to CUDA/HIP __global__
functions.

Differential Revision: https://reviews.llvm.org/D47958

llvm-svn: 334561