Rollup merge of #120739 - lukas-code:pp-dyn-assoc, r=compiler-errors

improve pretty printing for associated items in trait objects

* Don't print a binder in front of associated items, because it's not valid syntax.
  * e.g. print `dyn for<'a> Trait<'a, Assoc = &'a u8>` instead of `dyn for<'a> Trait<'a, for<'a> Assoc = &'a u8>`.
* Don't print associated items that are implied by a supertrait bound.
  * e.g. if we have `trait Sub: Super<Assoc = u8> {}`, then just print `dyn Sub` instead of `dyn Sub<Assoc = u8>`.

I've added the test in the first commit, so you can see the diff of the compiler output in the second commit.

Author: matthiaskrgr