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

[Callum Fare via llvm-commits]

callumfare wrote:

I don't mind the kernel being an opaque handle like the other types as opposed to `void*` like it is now. If the underlying pointer is useful to end-users we can expose it with something like `olGetKernelNativeHandle`. Long-term we'll need functions like that for every handle type anyway to support SYCL's native interop functionality.

I don't think we should have any state kept in the kernel (same reason we don't set arguments like UR/OpenCL) so I'm not a fan of any option where we store `DynMemorySize` in the kernel. Likewise adding a `ol_kernel_invocation` type seems overkill.

Option 2 seems best if we change `ol_kernel_handle_t`, otherwise option 1 seems fine.

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