E0451 needs to be updated to new format

[sophiajt]

From: src/test/compile-fail/E0451.rs

E0451 needs a span_label, updating it from:

error[E0451]: field `b` of struct `Bar::Foo` is private
  --> src/test/compile-fail/E0451.rs:19:13
   |
19 |     let f = Bar::Foo{ a: 0, b: 0 }; //~ ERROR E0451
   |             ^^^^^^^^^^^^^^^^^^^^^^

To:

error[E0451]: field `b` of struct `Bar::Foo` is private
  --> src/test/compile-fail/E0451.rs:19:13
   |
19 |     let f = Bar::Foo{ a: 0, b: 0 }; //~ ERROR E0451
   |             ^^^^^^^^^^^^^^^^^^^^^^ field `b` is private

Bonus (though this may be tricky): just underline the private field

error[E0451]: field `b` of struct `Bar::Foo` is private
  --> src/test/compile-fail/E0451.rs:19:13
   |
19 |     let f = Bar::Foo{ a: 0, b: 0 }; //~ ERROR E0451
   |                             ^^^^ field `b` is private