Rust: Refine implSiblings

[hvitved]

#21206 follow-up.

Consider two implementations of the same trait to be siblings when the type being implemented by one is an instantiation of the type being implemented by the other.

For example, in

trait MyTrait<T> { ... }
impl MyTrait<i64> for i64 { ... }    // I1
impl MyTrait<u64> for i64 { ... }    // I2

impl MyTrait<i64> for S<i64> { ... } // I3
impl MyTrait<u64> for S<u64> { ... } // I4
impl MyTrait<bool> for S<T> { ... }  // I5

the pairs (I1, I2), (I3, I5), and (I4, I5) are siblings, but not (I3, I4).