E0232 needs to be updated to new format

[sophiajt]

From: src/test/compile-fail/E0232.rs

E0232 needs a span_label, updated title, and a note, updating it from:

error[E0232]: this attribute must have a value, eg `#[rustc_on_unimplemented = "foo"]`
  --> src/test/compile-fail/E0232.rs:13:1
   |
13 | #[rustc_on_unimplemented] //~ ERROR E0232
   | ^^^^^^^^^^^^^^^^^^^^^^^^^

to:

error[E0232]: this attribute must have a value
  --> src/test/compile-fail/E0232.rs:13:1
   |
13 | #[rustc_on_unimplemented] //~ ERROR E0232
   | ^^^^^^^^^^^^^^^^^^^^^^^^^ attribute requires a value
   |
   = note: eg `#[rustc_on_unimplemented = "foo"]`