E0120 needs to be updated to new format

[sophiajt]

From: src/test/compile-fail/E0120.rs

Error E0120 needs a span_label, updating it from:

error[E0120]: the Drop trait may only be implemented on structures
  --> src/test/compile-fail/E0120.rs:13:1
   |
13 | impl Drop for MyTrait { //~ ERROR E0120
   | ^

To:

error[E0120]: the Drop trait may only be implemented on structures
  --> src/test/compile-fail/E0120.rs:13:1
   |
13 | impl Drop for MyTrait { //~ ERROR E0120
   | ^ implementing Drop requires a struct

Bonus: update the span to highlight the type.

error[E0120]: the Drop trait may only be implemented on structures
  --> src/test/compile-fail/E0120.rs:13:1
   |
13 | impl Drop for MyTrait { //~ ERROR E0120
   |               ^^^^^^^ implementing Drop requires a struct