Auto merge of #65013 - petertodd:2019-maybeuninit-debug, r=sfackler

Implement Debug for MaybeUninit

Precedent: `UnsafeCell` implements `Debug` even though it can't actually display the value. I noticed this omission while writing the following:

```
#[derive(Debug)]
 pub struct SliceInitializer<'a, T> {
    marker: PhantomData<&'a mut T>,
    uninit: &'a mut [MaybeUninit<T>],
    written: usize,
}
```

...which currently unergonomically fails to compile.

`UnsafeCell` does require `T: Debug`. Because of things like the above I think it'd be better to leave that requirement off. In fact, I'd also suggest removing that requirement for `UnsafeCell` too, which again I noticed in some low-level real world code.

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