fix: Improve inserted order for trait_impl_redundant_assoc_item

[A4-Tacks]

Example

trait Marker {
    fn foo();
    fn baz();
}
impl Marker for Foo {
    fn foo() {}
    fn missing() {}$0
    fn baz() {}
}

Before this PR

trait Marker {
    fn missing();
    fn foo();
    fn baz();
}
impl Marker for Foo {
    fn foo() {}
    fn missing() {}
    fn baz() {}
}

After this PR

trait Marker {
    fn foo();
    fn missing();
    fn baz();
}
impl Marker for Foo {
    fn foo() {}
    fn missing() {}
    fn baz() {}
}

[ChayimFriedman2]

I'm not sure we want this complexity. This is a small thing and if the items aren't in the same order as the trait it's not helpful anyway.

@rust-lang/rust-analyzer thoughts?

[flodiebold]

IMO it's worth it, and the complexity is pretty well-contained.

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.