remove AliasTy::def_id() this is part of https://github.com/rust-lang/rust/issues/156181 as well as part of https://github.com/rust-lang/rust/issues/152245 this immediately uses [the recently-introduced `Alias<>` type](https://github.com/rust-lang/rust/pull/156538) to narrow the kinds of aliases allowed in various places in code fyi @lcnr r? @BoxyUwU