Rollup merge of #80764 - CAD97:weak-unsized-as-ptr-again, r=RalfJung

Re-stabilize Weak::as_ptr and friends for unsized T

As per [T-lang consensus](https://hackmd.io/7r3_is6uTz-163fsOV8Vfg), this uses a branch to handle the dangling case. The discussed optimization of only doing the branch in the T: ?Sized case is left for a followup patch, as doing so is not trivial (as it requires specialization) and not _obviously_ better (as it requires using `wrapping_offset` rather than `offset` more).

<details><summary>Basically said optimization</summary>

Specialize on `T: Sized`:

```rust
fn as_ptr(&self) -> *const T {
    if [ T is Sized ] || !is_dangling(ptr) {
        (ptr as *mut T).set_ptr_value( (ptr as *mut u8).wrapping_offset(data_offset) )
    } else {
        ptr::null()
    }
}

fn from_raw(*const T) -> Self {
    if [ T is Sized ] || !ptr.is_null() {
        let ptr = (ptr as *mut RcBox).set_ptr_value( (ptr as *mut u8).wrapping_offset(-data_offset) );
        Weak { ptr }
    } else {
        Weak::new()
    }
}
```

(but with more `set_ptr_value` to avoid `Sized` restrictions and maintain metadata.)

Written in this fashion, this is not a correctness-critical specialization (i.e. so long as `[ T is Sized ]` is false for unsized `T`, it can be `rand()` for sized `T` without breaking correctness), but it's still touchy, so I'd rather do it in another PR with separate review.

---
</details>

This effectively reverts #80422 and re-establishes #74160. T-libs [previously signed off](https://github.com/rust-lang/rust/pull/74160#issuecomment-660539373) on this stable API change in #74160.

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