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

[Tom Honermann via cfe-commits]

tahonermann wrote:

@erichkeane, it would be great if you could spare a few minutes to look over the addition of `clang/include/clang/AST/SubobjectVisitor.h` before we merge this PR.

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