We delay projection normalization to further stages in order to
register user type annotations before normalization in HIR typeck.

There are two consumers of astconv: ItemCtxt and FnCtxt.
The former already expects unnormalized types from astconv, see its
AstConv trait impl.
The latter needs `RawTy` for a cleaner interface.

Unfortunately astconv still needs the normalization machinery in
order to resolve enum variants that have projections in the self type,
e.g. `<<T as Trait>::Assoc>::StructVariant {}`.
This is why `AstConv::normalize_ty_2` is necessary.

Projection types in user annotations may contain inference variables.
This makes the normalization depend on the unification with the actual
type and thus requires a separate TypeOp to track the obligations.
Otherwise simply calling `TypeChecker::normalize` would ICE with
"unexpected ambiguity"