Rollup merge of #52834 - matthewjasper:allow-zst-conflicts, r=pnkfelix

[NLL] Allow conflicting borrows of promoted length zero arrays

This is currently overkill as there's no way to create two conflicting borrows of any promoted.
It is possible that the following code might not fail due to const eval in the future (@oli-obk?). In which case either the array marked needs to not be promoted, or to be checked for conflicts

```rust
static mut A: () = {
    let mut y = None;
    let z;
    let mut done_y = false;
    loop {
        let x = &mut [1];  // < this array
        if done_y {
            z = x;
            break;
        }
        y = Some(x);
        done_y = true;
    }
    some_const_fn(y, z); // some_const_fn expects that y to not alias z.
};
```

r? @pnkfelix  @nikomatsakis

closes #52671
cc #51823

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