Auto merge of #4365 - lukas-code:async_new_ret_no_self, r=flip1995

new_ret_no_self: allow Self in inner type for impl Trait return types

Check the inner types of associated types of a trait when checking for Self in the return type of a `new` method. This means that the following will no longer warn:
```rust
trait Trait {
    type Inner;
}

struct S;

impl S {
    fn new() -> impl Trait<Inner = Option<Self>> {
        struct TraitImpl;

        impl Trait for TraitImpl {
            type Inner = Option<S>;
        }

        TraitImpl
    }
}
```
```rust
#![feature(async_await)]

struct Connection;

impl Connection {
    async fn new() -> Result<Self, ()> {
        Ok(S)
    }
}
```
closes #4359

changelog: fix `new_ret_no_self` lint for async `new` functions.

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