Rollup merge of #129668 - coolreader18:fix-pin-set-regr, r=dtolnay

Fix Pin::set bounds regression

Fixes #129601

Fixes the regression from #129449, where changing the bounds of the impl block containing `Pin::set` changed the method resolution behavior.

```rust
struct A;
impl A {
    fn set(&self) {}
}

let a: Pin<&A>;
a.set();
// before:
// - checks <impl<Ptr: DerefMut> Pin<Ptr>>::set(): `&A` doesn't impl `DerefMut`
// - autorefs -> &A: resolves to A::set()
// now:
// - checks <impl<Ptr: Deref> Pin<Ptr>>::set(): `&A` impls `Deref`! resolves to Pin::set()
// - check method bounds: `&A` doesn't impl DerefMut: error
```

r? `@dtolnay`

Author: matthiaskrgr