Internal Tracking Issue for the `AliasTerm` refactor

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

About tracking issues

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Steps

• move DefId of type aliases into AliasTyKind https://github.com/rust-lang/rust/pull/154758
• do the same for constants, updating AliasTerm https://github.com/rust-lang/rust/pull/155392
• https://github.com/rust-lang/rust/pull/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

No files or tests are named. Start by reviewing the unchecked checklist items and the linked pull requests for the completed AliasTerm changes, then trace the remaining TypeError and mk_ty_from_kind work in the compiler. Done means the specific constructor replacement is complete and TypeError uses the more specific DefIds described by the issue.