default auto traits: use default supertraits instead of `Self: Trait` bounds on associated items
First commit: the motivation has been discussed [here](https://github.com/rust-lang/rust/pull/144679).
Second commit: the only new places where new implicit `DefaultAutoTrait` bounds are generated are supertraits and trait object so `?Trait` syntax should be extended to these places only.
r? `@lcnr`
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