[llvm] [Offload] Allow setting null arguments in olLaunchKernel and fix amdgpu handling for empty kernel arguments (PR #141958)

[Ross Brunton via llvm-commits]

RossBrunton wrote:

I've pushed up an additional change to this PR; the thing I was debugging originally was that the amdgpu driver would throw an error if a function was being called with no arguments at all.

However, more digging has revealed that even if you build a kernel with no arguments at the C level, when compiled at O0 a number of "hidden" arguments are added. The new change compiles the noargs.cpp test with O3 and fixes amdgpu to pass.

Not sure if the fixes to amdgpu are worth moving into their own MR or not though.

Also, I checked: `kernarg_address` in `hsa_kernel_dispatch_packet_t` may be `NULL`.

https://github.com/llvm/llvm-project/pull/141958