Don't suggest changing &self and &mut self in function signature to be mutable when taking &mut self in closure

[jieyouxu]

Current suggestion for when taking a mutable reference to self in a closure (as an upvar) will produce a machine-applicable suggestion to change the self in the function signature to mut self, but does not account for the specialness of implicit self in that it can already have & and &mut (see #111554). This causes the function signature to become test(&mut mut self) which does not seem desirable.

error[E0596]: cannot borrow `self` as mutable, as it is not declared as mutable
   --> src/sound_player.rs:870:11
    |
869 |     pub fn test(&mut self) {
    |                      ---- help: consider changing this to be mutable: `mut self`
870 |     || test2(&mut self);
    |              ^^^^^^^^^ cannot borrow as mutable

This PR suppresses the "changing this to be mutable" suggestion if the implicit self is either ImplicitSelfKind::ImmRef or ImplicitSelfKind::MutRef.

Fixes #111554.

[rustbot]

r? @jackh726

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[jieyouxu]

@rustbot ready

[compiler-errors]

@bors r+ rollup

[bors]

📌 Commit 57e67e4 has been approved by compiler-errors

It is now in the queue for this repository.