`min-type-alias-impl-trait` should not accept impl trait in assoc. type binding

[nikomatsakis]

The following example represents an illegal use for min_type_alias_impl_trait

playground of more complete code

pub type FooImpl = impl Foo;
pub type BarImpl = impl Bar<Foo = FooImpl>;

But we are not reporting it that way.

[oli-obk]

It seems fine to me under the full type-alias-impl-trait feature as implemented. I think we should just close this issue and keep allowing the example

[nikomatsakis]

This works "as expected" now and we have tests, @oli-obk claims :)