Auto merge of #86848 - notriddle:notriddle/drop-dyn, r=varkor

feat(rustc_lint): add `dyn_drop`

Based on the conversation in #86747.

Explanation
-----------

A trait object bound of the form `dyn Drop` is most likely misleading and not what the programmer intended.

`Drop` bounds do not actually indicate whether a type can be trivially dropped or not, because a composite type containing `Drop` types does not necessarily implement `Drop` itself. Naïvely, one might be tempted to write a deferred drop system, to pull cleaning up memory out of a latency-sensitive code path, using `dyn Drop` trait objects. However, this breaks down e.g. when `T` is `String`, which does not implement `Drop`, but should probably be accepted.

To write a trait object bound that accepts anything, use a placeholder trait with a blanket implementation.

```rust
trait Placeholder {}
impl<T> Placeholder for T {}
fn foo(_x: Box<dyn Placeholder>) {}
```

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