Rollup merge of #115347 - y21:generic-bound-impl-trait-ty, r=compiler-errors

suggest removing `impl` in generic trait bound position

rustc already does this recovery in type param position (`<T: impl Trait>` -> `<T: Trait>`).
This PR also adds that suggestion in trait bound position (e.g. `where T: impl Trait` or `trait Trait { type Assoc: impl Trait; }`)

Author: matthiaskrgr