Support unresolved type alias as well

[zgrannan]

Don't add outlives constraints, even if assoc types have lifetime params:

Aureas examples (for reference):

trait Foo<'a> {
    type Assoc<'b> where 'a: 'b, Self: 'b;
    fn get<'b>(&'b mut self) -> Self::Assoc<'b>;
}

struct A<'a> {
    f: &'a mut i32,
}

impl<'a> Foo<'a> for A<'a> {
    type Assoc<'b> = &'b mut i32
         where 'a: 'b, Self: 'b;
    fn get<'b>(&'b mut self) -> Self::Assoc<'b> {
        &mut self.f
    }
}

fn main() {
    let mut i = 42;
    let mut a = A {
        f: &mut i,
    };
    *a.get() += 1;
    println!("{i}");
}

[zgrannan]

Resolved by #156