Struct literal completion should also suggest `Self { ... }` when in an impl

[jonas-schievink]

This suggests Strukt {...}:

struct Strukt {
    a: u8,
}

impl Strukt {
    fn new() -> Strukt {
        Strukt$0
    }
}

This does not suggest Self {...}:

struct Strukt {
    a: u8,
}

impl Strukt {
    fn new() -> Strukt {
        Self$0
    }
}