Prettify E0379

Original issue #35338, PR #35364.

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

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
```
