Add nuance to suggestion of !Copy expressions in for loops

[estebank]

Given

struct S(i32);
fn foo(x: Vec<S>) {
    for y in x {
        
    }
    let z = x;
}

we currently suggest borrowing x:

error[E0382]: use of moved value: `x`
   --> src/lib.rs:6:13
    |
2   | fn foo(x: Vec<S>) {
    |        - move occurs because `x` has type `Vec<S>`, which does not implement the `Copy` trait
3   |     for y in x {
    |              -
    |              |
    |              `x` moved due to this implicit call to `.into_iter()`
    |              help: consider borrowing to avoid moving into the for loop: `&x`
...
6   |     let z = x;
    |             ^ value used here after move
    |
note: this function takes ownership of the receiver `self`, which moves `x`

The note doesn't have a span, so we should remove it when it is DUMMY_SP.

When we make the y pattern mut, we get the same output.

We should likely suggest .iter_mut() when the pattern is mutable, and the expression isn't Copy.

[estebank]

Current output:

error[[E0382]](https://doc.rust-lang.org/stable/error_codes/E0382.html): use of moved value: `x`
 --> src/lib.rs:8:13
  |
4 | fn foo(x: Vec<S>) {
  |        - move occurs because `x` has type `Vec<S>`, which does not implement the `Copy` trait
5 |     for y in x {
  |              - `x` moved due to this implicit call to `.into_iter()`
...
8 |     let z = x;
  |             ^ value used here after move
  |
note: `into_iter` takes ownership of the receiver `self`, which moves `x`
 --> /rustc/8ede3aae28fe6e4d52b38157d7bfe0d3bceef225/library/core/src/iter/traits/collect.rs:271:18
help: consider iterating over a slice of the `Vec<S>`'s content to avoid moving into the `for` loop
  |
5 |     for y in &x {
  |              +