Add new error comparision to hide desugaring

Add new error comparision to hide desugaring

First some background:
To the compiler, the following two signatures in the trait vs the impl
are the same.

```rust
trait Foo {
    fn foo(&self, &impl Debug);
}
impl Foo for () {
    fn foo<U: Debug>(&self, x: &U) { ... }
}
```

We do not want to allow this, and so we add a new error and check.

The check just tests that all paramters 'syntheticness' match up. As
during collection, the impl Trait parameters are transformed into
anonymous synthetic generics.

Furthermore, causes a check for unused type parameters to be skipped in
check_bounds_are_used if there is already a TyError. Thus, an unused
input will not trigger `type parameter unused` errors.

Update the one test that checked for this error in the case of
a TyError.