Improve error message for reserved ambiguation

[npmccallum]

Compiling this code:

trait Foo {
    type not_really_ambiguous_i_promise;
}

impl Foo for isize {
    type not_really_ambiguous_i_promise = usize;
}

fn main() {
    let x: isize::not_really_ambiguous_i_promise = 17;
    println!("{}", x);
}

Produces this error:

error[E0223]: ambiguous associated type
  --> src/main.rs:10:12
   |
10 |     let x: isize::not_really_ambiguous_i_promise = 17;
   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ambiguous associated type
   |
   = note: specify the type using the syntax `<isize as Trait>::not_really_ambiguous_i_promise`

Presuming that this is because such code might become ambiguous in the future, we need a better error message.

[oli-obk]

The diagnostic is effectively unchanged.

there should be a help stating something around the fact that other traits may have an assoc type of the same name, and thus adding an impl for such a type will become a breaking change.