Rollup merge of #138517 - compiler-errors:better-child-capture, r=oli-obk

Improve upvar analysis for deref of child capture

Two fixes to the heuristic I implemented in #123660. As I noted in the code:

> Luckily, if this function is not correct, then the program is not unsound, since we still borrowck and validate the choices made from this function -- the only side-effect is that the user may receive unnecessary borrowck errors.

This indeed fixes unnecessary borrowck errors.

r? oli-obk

---

The heuristic is only valid if we deref a `&T`, not a `&mut T` or `Box<T>`, so make sure to check the type. This fixes:

```rust
struct Foo { precise: i32 }

fn mut_ref_inside_mut(f: &mut Foo) {
    let x: impl AsyncFn() = async move || {
        let y = &f.precise;
    };
}
```

Since the capture from `f` to `&f.precise` needs to be treated as a lending borrow from the parent coroutine-closure to the child coroutine.

---

The heuristic is also valid if *any* deref projection in the child capture's projections is a `&T`, but we were only looking at the last one. This ensures that this function is considered not to be lending:

```rust
struct Foo { precise: i32 }

fn ref_inside_mut(f: &mut &Foo) {
    let x: impl Fn() -> _ = async move || {
        let y = &f.precise;
    };
}
```

(Specifically, checking that `impl Fn() -> _` is satisfied is exercising that the coroutine is not considered to be lending.)

Author: jhpratt