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

[Ross Brunton via llvm-commits]

RossBrunton wrote:

@jhuber6 The max work group size is dependant on the amount of dynamic memory the kernel will launch with, so we need to have that passed in as well.

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