[copybara-service]

Make unique_ptr::as_pin and virtual_unique_ptr::as_pin panic on null instead of returning Option.

With nonnull_smart_pointers, nullable C++ std::unique_ptrs bind in Rust as Nullable<unique_ptr<T>> (or Nullable<virtual_unique_ptr<T>>), while bare unique_ptr<T> and virtual_unique_ptr<T> represent pointers that are non-null unless moved-from or constructed via from_raw(null). Deref, DerefMut, as_mut, into_box, into_pin_box, and into_inner already panic on null; only as_pin returned Option<Pin<&mut T>>, forcing every !Unpin caller to unwrap a None that cannot occur.

Change unique_ptr::as_pin and virtual_unique_ptr::as_pin to return Pin<&mut T> directly and panic on null ("dereferencing a null unique_ptr" / "dereferencing a null virtual_unique_ptr"), matching as_mut and Deref. Callers that legitimately hold a possibly-null bare unique_ptr can check with unique_ptr::is_null first, or use Nullable<unique_ptr<T>>.