Add UnsafeBinder variant (WIP in rustc).

Rustc has started implementing `UnsafeBinder`:
- rust-lang/rust#134625
- rust-lang/rust#130516
Only the enums variants have been added for now, semantics is TBD.

kani-compiler/src/codegen_cprover_gotoc/codegen/typ.rs

@@ -625,6 +625,8 @@ impl<'tcx> GotocCtx<'tcx> {
             ty::CoroutineWitness(_, _) | ty::Infer(_) | ty::Placeholder(_) | ty::Error(_) => {
                 unreachable!("remnants of type checking")
             }
+            // TODO cf. https://github.com/rust-lang/rust/issues/130516
+            ty::UnsafeBinder(_) => todo!(),
         }
     }
 
@@ -1032,6 +1034,8 @@ impl<'tcx> GotocCtx<'tcx> {
             ty::Infer(_) => todo!("{:?} {:?}", pointee_type, pointee_type.kind()),
             ty::Param(_) => todo!("{:?} {:?}", pointee_type, pointee_type.kind()),
             ty::Placeholder(_) => todo!("{:?} {:?}", pointee_type, pointee_type.kind()),
+            // TODO cf. https://github.com/rust-lang/rust/issues/130516
+            ty::UnsafeBinder(_) => todo!(),
         }
     }