E0128 needs to be updated to new format

[sophiajt]

From: src/test/compile-fail/E0128.rs

Error E0128 needs a span_label, updating it from:

error[E0128]: type parameters with a default cannot use forward declared identifiers
  --> src/test/compile-fail/E0128.rs:11:14
   |
11 | struct Foo<T=U, U=()> { //~ ERROR E0128
   |              ^

To:

error[E0128]: type parameters with a default cannot use forward declared identifiers
  --> src/test/compile-fail/E0128.rs:11:14
   |
11 | struct Foo<T=U, U=()> { //~ ERROR E0128
   |              ^ defaulted type parameters cannot be forward declared

[circuitfox]

Working on this now.