Auto merge of #77893 - petertodd:2020-impl-default-for-phantompinned, r=dtolnay

Impl Default for PhantomPinned

`PhantomPinned` is just a marker type, with an obvious default value (the only value). So I can't think of a reason not to do this. Sure, it's used in exotic situations with unsafe code. But the people writing that code can decide for themselves if they can derive `Default`, and in many situations the derived impl will make sense:

```rust
#[derive(Default)]
struct NeedsPin {
   marker: PhantomPinned,
   buf: [u8; 1024],
   ptr_to_data: Option<*const u8>,
}
```

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