Auto merge of #134625 - compiler-errors:unsafe-binders-ty, r=oli-obk

Begin to implement type system layer of unsafe binders

Mostly TODOs, but there's a lot of match arms that are basically just noops so I wanted to split these out before I put up the MIR lowering/projection part of this logic.

r? oli-obk

Tracking:

- https://github.com/rust-lang/rust/issues/130516

compiler/rustc_smir/src/rustc_smir/convert/ty.rs

@@ -356,6 +356,8 @@ impl<'tcx> Stable<'tcx> for ty::TyKind<'tcx> {
             ty::FnPtr(sig_tys, hdr) => {
                 TyKind::RigidTy(RigidTy::FnPtr(sig_tys.with(*hdr).stable(tables)))
             }
+            // FIXME(unsafe_binders):
+            ty::UnsafeBinder(_) => todo!(),
             ty::Dynamic(existential_predicates, region, dyn_kind) => {
                 TyKind::RigidTy(RigidTy::Dynamic(
                     existential_predicates