Rollup merge of #148491 - estebank:issue-147894, r=jackh726

 Correctly provide suggestions when encountering `async fn` with a `dyn Trait` return type

Point at return type of `async fn`s instead of their def `Span`.

Correctly provide suggestions when encountering `async fn` with a `dyn Trait` return type.

Correctly address `type Alias = dyn Trait` when `fn foo() -> Alias { .. }`.

Fix rust-lang/rust#147894.

Author: matthiaskrgr