Replace hir::TyImplTrait with TyImplTraitUniversal and
TyImplTraitExistential.
Add an ImplTraitContext enum to rustc::hir::lowering to track the kind
and allowedness of an impl Trait.
Significantly alter lowering to thread ImplTraitContext and one other
boolean parameter described below throughought much of lowering.
The other parameter is for tracking if lowering a function is in a trait
impl, as there is not enough information to otherwise know this
information during lowering otherwise.
This change also removes the checks from ast_ty_to_ty for impl trait
allowedness as they are now all taking place in HIR lowering.
8fd48e7d59