E0084 needs to be updated to new format

[sophiajt]

From: src/test/compile-fail/E0084.rs

E0084 needs a span_label, updating it from:

error[E0084]: unsupported representation for zero-variant enum
  --> src/test/compile-fail/E0084.rs:12:1
   |
12 | enum Foo {} //~ ERROR E0084
   | ^^^^^^^^^^^

To:

error[E0084]: unsupported representation for zero-variant enum
  --> src/test/compile-fail/E0084.rs:12:1
   |
12 | enum Foo {} //~ ERROR E0084
   | ^^^^^^^^^^^ unsupported enum representation