[llvm] [Offload] Add olGetKernelMaxGroupSize (PR #142950)

[Ross Brunton via llvm-commits]

RossBrunton wrote:

@jhuber6 I'm still trying to wrap my head around what you are looking for here. olGetKernelInfo or even having ol_kernel_handle_t be a non-opaque type makes sense for other queries, but the max group size depends on the local memory size (which as far as I can tell only the caller knows about).

We could have a dedicated `olGetKernelLaunchInfo` (or similar) that accepts a kernel, device and local memory size and populates a provided struct with the work group size, but I don't know what information other than the max WG size such a struct could contain.

https://github.com/llvm/llvm-project/pull/142950