[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


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@328349 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed