Handle Sized? in type items.

Resolves bounds for `type` and adds the warning for 'unbounds' (? bounds) that we have for bounds.

Closes #16888

src/test/run-pass/unsized.rs

@@ -24,6 +24,7 @@ struct S1<Sized? X>;
 enum E<Sized? X> {}
 impl <Sized? X> T1 for S1<X> {}
 fn f<Sized? X>() {}
+type TT<Sized? T> = T;
 
 pub fn main() {
 }