Extend async fn test suite to account for ref mut? patterns

[Centril]

In #60501 the handling of mut patterns in arguments is fixed (#60498).

However, the test suite should also be extended to account for:

type A = Vec<u32>;

async fn foo(ref mut vec: A) { ... }

and:

async fn foo(ref vec: A) { ... }

as well as nestings of those:

async fn foo((ref mut c, ref d): (A, A)) { ... }

async fn foo((a, mut b): (A, A)) { ... }

Please be creative in testing for combinatorial interactions when closing this issue :)

[matthewjasper]

type A = Vec<u32>;

async fn foo(ref mut vec: A) { ... }

This appears to be broken on nightly, desugaring into:

fn foo(ref mut vec: A) {
    static move || {
        let vec = vec; // let ref mut vec = vec; once #60501 is merged, AFAICT
    }   
}

Which the borrow checker complains about, because the generator is now effective capturing by-ref.

The correct lowering is:

fn foo(__arg1: A) {
    static move || {
        let __arg1 = __arg1; 
        let ref mut vec = __arg1;
    }   
}

[taiki-e]

This appears to be broken too:

async fn foo((ref mut c, ref d): (A, A)) { ... }

Error:

error[E0596]: cannot borrow `__arg0.0` as mutable, as `__arg0` is not declared as mutable
  --> code\src\lib.rs:17:15
   |
17 | async fn foo((ref mut c, ref d): (A, A)) {}
   |              -^^^^^^^^^--------
   |              ||
   |              |cannot borrow as mutable
   |              help: consider changing this to be mutable: `mut __arg0`

[taiki-e]

In addition, the following code gets the wrong warning because I forgot to remove mut from inputs in #60501 (I overlooked the warning being ignored in the test).

async fn bar(n: u32, mut vec: Vec<u32>) {
    vec.push(n)
}

Warning:

warning: variable does not need to be mutable
 --> code\src\lib.rs:8:22
  |
8 | async fn bar(n: u32, mut vec: Vec<u32>) {
  |                      ----^^^
  |                      |
  |                      help: remove this `mut`
  |
  = note: #[warn(unused_mut)] on by default

[taiki-e]

Opened #60535 to close this issue and to fix all the errors/warnings reported here