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
83c431fb9e