[AMDGPU] Remove use of OpenCL triple environment and replace with function attribute for AMDGPU

- Remove use of the opencl and amdopencl environment member of the target triple for the AMDGPU target.
- Use function attribute to communicate to the AMDGPU backend to add implicit arguments for OpenCL kernels for the AMDHSA OS.

Differential Revision: https://reviews.llvm.org/D43736

llvm-svn: 328349