[`len_without_is_empty`]: follow type alias to find inherent `is_empty` method
Fixes#11165
When we see an `impl B` and `B` is a type alias to some type `A`, then we need to follow the type alias to look for an `is_empty` method on the aliased type `A`. Before this PR, it'd get the inherent impls of `B`, which there aren't any and so it would warn that there isn't an `is_empty` method even if there was one.
Passing the type alias `DefId` to `TyCtxt::type_of` gives us the aliased `DefId` (or simply return the type itself if it wasn't a type alias) so we can just use that
changelog: [`len_without_is_empty`]: follow type alias to find inherent `is_empty` method
