Extract type as type alias assist does not fill infer vars

[ChayimFriedman2]

This is also true for non-lifetime generic parameters:

fn main() {
    let vec: Vec<_> = vec![3];
    dbg!(vec);
}

becomes:

type Type = Vec<_>;

fn main() {
    let vec: Type = vec![3];
    dbg!(vec);
}

Originally posted by @nicolas-guichard in #20107

[Hmikihiro]

Would it make sense to generate a type parameter instead of using a wildcard?

type Type<A> = Vec<A>;

fn main() {
    let vec: Type<_> = vec![3];
    dbg!(vec);
}

[ChayimFriedman2]

I feel it's better to use the inferred type.

[nicolas-guichard]

I initially thought that we should generate a type parameter (like required by #20107), but in the end I agree that using the inferred type is better.

This extreme case convinced me:

fn main() {
    let vec: _ = vec![3];
    dbg!(vec);
}

We definitely want to extract type Type = Vec<i32>; and not type Type<T> = T; here.