introduce a fresh variant for "non-normalized" associated types

[nikomatsakis]

In the compiler today, we have one variant for associated type projections (e.g., T::Foo):

rust/src/librustc/ty/sty.rs

Lines 156 to 158 in 727eabd

	/// The projection of an associated type. For example,
	/// `<T as Trait<..>>::N`.
	Projection(ProjectionTy<'tcx>),

We use context to know whether this is normalized or not -- basically, it should always be normalized, unless the type results from one of those queries that fetches the type that the user wrote for something like a field etc. In the lazy normalization approach that Chalk uses, however, we would want to be able to explicitly represent this difference.

The job here is to add a new variant. I think we should call it TyUnnormalizedProjection. This would be entirely unused for now -- but when we start to enable chalk, we will create this type when translating user types directly, and only create TyProjection in the case that we have attempted to normalize the projection and failed.

cc @tmandry @scalexm

[scalexm]

Introduced in #54789.