This is a tracking issue for a refactoring of the way aliases are represented in the type system.

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Steps

• move DefId of type aliases into AliasTyKind ty::Alias refactor #154758
• do the same for constants, updating AliasTerm AliasTerm refactor #155392
• Use special DefIds for aliases #155981
• merge AliasTy and AliasTerm and have a single Alias type generic over the kind.
• remove AliasTerm::def_id() and AliasTy::def_id as not all variants will ahve DefIds in the future, instead track the current variant more explicitly
  • TypeError use more specific DefIds, requires change to Relate impl of AliasTy
• replace uses of tcx.mk_ty_from_kind with calls to a more specific constructor method