[amdgpu] Add amdgpu_kernel calling conv attribute to clang

Allows emitting define amdgpu_kernel void @func() IR from C or C++.

This replaces the current workflow which is to write a stub in opencl that
calls an external C function implemented in C++ combined through llvm-link.

Calling the resulting function still requires a manual implementation of the
ABI from the host side. The primary application is for more rapid debugging
of the amdgpu backend by permuting a C or C++ test file instead of manually
updating an IR file.

Implementation closely follows D54425. Non-amd reviewers from there.

Reviewed By: yaxunl

Differential Revision: https://reviews.llvm.org/D125970