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

[Tom Honermann via cfe-commits]

================
@@ -665,6 +666,101 @@ OutlinedFunctionDecl *BuildSYCLKernelEntryPointOutline(Sema &SemaRef,
   return OFD;
 }
 
+class KernelArgsChecker : public SubobjectVisitor<KernelArgsChecker> {
----------------
tahonermann wrote:

I think `KernelArgsChecker` should be able to derive from `ConstSubobjectVisitor`; we shouldn't need to modify any objects. If doing so runs into problems because of const correctness issues elsewhere though, it probably isn't worth trying to change.

```suggestion
class KernelArgsChecker : public ConstSubobjectVisitor<KernelArgsChecker> {
```

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