Rollup merge of #136032 - estebank:issue-136028, r=SparrowLii

Account for mutable borrow in argument suggestion

```
error: value assigned to `object` is never read
  --> $DIR/mut-arg-of-borrowed-type-meant-to-be-arg-of-mut-borrow.rs:21:5
   |
LL |     object = &mut object2;
   |     ^^^^^^
   |
help: you might have meant to mutate the pointed at value being passed in, instead of changing the reference in the local binding
   |
LL ~ fn change_object3(object: &mut Object) {
LL |
LL |     let object2 = Object;
LL ~     *object = object2;
   |
```
instead of
```
error: value assigned to `object` is never read
  --> $DIR/mut-arg-of-borrowed-type-meant-to-be-arg-of-mut-borrow.rs:21:5
   |
LL |     object = &mut object2;
   |     ^^^^^^
   |
help: you might have meant to mutate the pointed at value being passed in, instead of changing the reference in the local binding
   |
LL ~ fn change_object3(object: &mut mut Object) {
LL |
LL |     let object2 = Object;
LL ~     *object = object2;
   |
```

Fix #136028.

compiler/rustc_passes/src/errors.rs

@@ -1791,7 +1791,7 @@ pub(crate) struct UnusedAssign {
 pub(crate) struct UnusedAssignSuggestion {
     pub pre: &'static str,
     #[suggestion_part(code = "{pre}mut ")]
-    pub ty_span: Span,
+    pub ty_span: Option<Span>,
     #[suggestion_part(code = "")]
     pub ty_ref_span: Span,
     #[suggestion_part(code = "*")]