E0379 needs to be updated to new format

[sophiajt]

From: src/test/compile-fail/const-fn-mismatch.rs

Error E0379 needs to a span_label, updating it from:

error[E0379]: trait fns cannot be declared const
  --> src/test/compile-fail/const-fn-mismatch.rs:23:5
   |
23 |     const fn f() -> u32 { 22 } //~ ERROR E0379
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^

to:

error[E0379]: trait fns cannot be declared const
  --> src/test/compile-fail/const-fn-mismatch.rs:23:5
   |
23 |     const fn f() -> u32 { 22 } //~ ERROR E0379
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ trait tns cannot be const

Bonus: shrink the span to focus on the const part:

error[E0379]: trait fns cannot be declared const
  --> src/test/compile-fail/const-fn-mismatch.rs:23:5
   |
23 |     const fn f() -> u32 { 22 } //~ ERROR E0379
   |     ^^^^^ trait tns cannot be const