[clang] [clang][SYCL] Diagnose reference kernel arguments (PR #192957)

[Tom Honermann via cfe-commits]

https://github.com/tahonermann requested changes to this pull request.

Thanks @Fznamznon, I think this is looking good. I see I gave you a bad suggestion for the implementation of `traverseRecord()` previously; I added several comments discussing that. Otherwise, these comments are getting into more nit-picky details. A couple of questions are included.

https://github.com/llvm/llvm-project/pull/192957