Fix associate type candidate collection

[sbillig]

Deduping by TyId would result in erroneous diagnostics when there are two trait impls for a given type, where the impls have associated types with the same name and type. For example, in this test case, there would be an error on the return type of Foo's implementation of fn sub.

trait Add<T = Self> {
    type Output = Self
    fn add(self: Self, _ rhs: T) -> Self::Output
}
trait Sub<T = Self> {
    type Output = Self
    fn sub(self: Self, _ rhs: T) -> Self::Output
}

struct Foo {}

impl Add for Foo {
    fn add(self: Self, _ rhs: Self) -> Self { self }
}
impl Sub for Foo {
    fn sub(self: Self, _ rhs: Self) -> Self { self }
}