Point to let when modifying field of immutable variable

Point at the immutable local variable when trying to modify one of its
fields.

Given a file:

```rust
struct Foo {
    pub v: Vec<String>
}

fn main() {
    let f = Foo { v: Vec::new() };
    f.v.push("cat".to_string());
}
```

present the following output:

```
error: cannot borrow immutable field `f.v` as mutable
 --> file.rs:7:13
  |
6 |    let f = Foo { v: Vec::new() };
  |        - this should be `mut`
7 |    f.v.push("cat".to_string());
  |    ^^^

error: aborting due to previous error
```

src/test/ui/did_you_mean/issue-39544.stderr

@@ -1,6 +1,8 @@
 error: cannot borrow immutable field `z.x` as mutable
   --> $DIR/issue-39544.rs:21:18
    |
+20 |     let z = Z { x: X::Y };
+   |         - this should be `mut`
 21 |     let _ = &mut z.x;
    |                  ^^^