trait_solver: normalizes_to todo! on UnsafeBinder for Pointee/DiscriminantKind

[SebTardif]

Bug

compiler/rustc_next_trait_solver/src/solve/normalizes_to.rs:

• consider_builtin_pointee_candidate: ty::UnsafeBinder(_) => todo!()
• consider_builtin_discriminant_kind_candidate: ty::UnsafeBinder(_) => todo!("discr subgoal...")

Origin

Unsafe binder type system work; incomplete solver support:

rust-lang@9a1c5eb5b38

Impact

ICE normalizing <unsafe<T> as Pointee>::Metadata or <unsafe<T> as DiscriminantKind>::Discriminant (feature-gated; part of #567).

Fix

Pointee: recurse through inner type via with_replaced_self_ty. Discriminant: use discriminant_ty which already handles UnsafeBinder inners.