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

[Ross Brunton via llvm-commits]

RossBrunton wrote:

@jhuber6 So something like option 2, but with `ol_symbol_handle_t` instead of `ol_kernel_handle_t`?

I think having an `olGetKernelMaxGroupSize` (distinct from `olGetSymbolInfo`) method is pretty much unavoidable if we don't want to have the symbol type store `DynamicMemorySize`.

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