Bug: missing drop impls for arrays

With `--add-drop-bounds`, arrays still give a `BuiltinOrAuto`. We should give them a proper `Drop` impl instead, and be more explicit about what a `BuiltinOrAuto` for `Drop` means (the remaining ones should all be no-ops).
```rust
fn nested_from_fn<const K: usize>() -> [[usize; K]; K] {
    core::array::from_fn(|j| core::array::from_fn(|i| i+j))
}
```

cc @msprotz