Suggest making binding mut on &mut reborrow

[estebank]

When a binding needs to be mutably reborrowed multiple times, suggesting removing &mut will lead to follow up errors. Instead suggest both making the binding mutable and removing the reborrow.

error[E0596]: cannot borrow `outer` as mutable, as it is not declared as mutable
 --> f14.rs:2:12
  |
2 |     match (&mut outer, 23) {
  |            ^^^^^^^^^^ cannot borrow as mutable
  |
note: the binding is already a mutable borrow
 --> f14.rs:1:16
  |
1 | fn test(outer: &mut Option<i32>) {
  |                ^^^^^^^^^^^^^^^^
help: consider making the binding mutable if you need to reborrow multiple times
  |
1 | fn test(mut outer: &mut Option<i32>) {
  |         +++
help: if there is only one mutable reborrow, remove the `&mut`
  |
2 -     match (&mut outer, 23) {
2 +     match (outer, 23) {
  |

Address #81059.

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