Rollup merge of #145879 - Bryanskiy:supertraits-2, r=lcnr

default auto traits: use default supertraits instead of `Self: Trait` bounds on associated items

First commit: the motivation has been discussed [here](rust-lang/rust#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`

Author: matthiaskrgr