E0017 needs to be updated to new format

[sophiajt]

From: src/test/compile-fail/E0017.rs

E0017 needs a span_label to be added, taking the examples from:

error[E0017]: references in constants may only refer to immutable values
  --> src/test/compile-fail/E0017.rs:14:30
   |
14 | const CR: &'static mut i32 = &mut C; //~ ERROR E0017
   |                              ^^^^^^

to:

error[E0017]: references in constants may only refer to immutable values
  --> src/test/compile-fail/E0017.rs:14:30
   |
14 | const CR: &'static mut i32 = &mut C; //~ ERROR E0017
   |                              ^^^^^^ constants require immutable values

Bonus: you'll notice this test file duplicates its error messages. Ideally, each error would be printed only once.