Show trait method signature when impl differs

When the trait's span is available, it is already being used, add a
`note` for the cases where the span isn't available:

```
error[E0053]: method `fmt` has an incompatible type for trait
  --> $DIR/trait_type.rs:17:4
   |
17 |    fn fmt(&self, x: &str) -> () { }
   |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ types differ in mutability
   |
   = note: expected type `fn(&MyType, &mut std::fmt::Formatter<'_>) -> std::result::Result<(), std::fmt::Error>`
              found type `fn(&MyType, &str)`

error[E0050]: method `fmt` has 1 parameter but the declaration in trait `std::fmt::Display::fmt` has 2
  --> $DIR/trait_type.rs:21:11
   |
21 |    fn fmt(&self) -> () { }
   |           ^^^^^ expected 2 parameters, found 1
   |
   = note: `fmt` from trait: `fn(&Self, &mut std::fmt::Formatter<'_>) -> std::result::Result<(), std::fmt::Error>`

error[E0186]: method `fmt` has a `&self` declaration in the trait, but not in the impl
  --> $DIR/trait_type.rs:25:4
   |
25 |    fn fmt() -> () { }
   |    ^^^^^^^^^^^^^^^^^^ expected `&self` in impl
   |
   = note: `fmt` from trait: `fn(&Self, &mut std::fmt::Formatter<'_>) -> std::result::Result<(), std::fmt::Error>`

error[E0046]: not all trait items implemented, missing: `fmt`
  --> $DIR/trait_type.rs:28:1
   |
28 | impl std::fmt::Display for MyType4 {}
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `fmt` in implementation
   |
   = note: `fmt` from trait: `fn(&Self, &mut std::fmt::Formatter<'_>) -> std::result::Result<(), std::fmt::Error>`
```

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