Rollup merge of #60300 - mjbshaw:ffi_types, r=rkruppe

Allow null-pointer-optimized enums in FFI if their underlying representation is FFI safe

I'm not sure if this requires an RFC. I attempted to start [a discussion on internals.rust-lang.org](https://internals.rust-lang.org/t/options-ffi-safety-and-guarantees-for-abi-compatibility-with-nonnull-optimizations/9784) and when no one really objected I figured I'd go ahead and try implementing this.

This allows types like `Option<NonZeroU8>` to be used in FFI without triggering the `improper_ctypes` lint. This works by changing the `is_repr_nullable_ptr` function to consider an enum `E` to be FFI-safe if:

- `E` has no explicit `#[repr(...)]`.
- It only has two variants.
- One of those variants is empty (meaning it has no fields).
- The other variant has only one field.
- That field is one of the following:
  - `&T`
  - `&mut T`
  - `extern "C" fn`
  - `core::num::NonZero*`
  - `core::ptr::NonNull<T>`
  - `#[repr(transparent)] struct` wrapper around one of the types in this list.
- The size of `E` and its field are both known and are both the same size (implying `E` is participating in the nonnull optimization).

This logic seems consistent with [the Rust nomicon](https://doc.rust-lang.org/nomicon/repr-rust.html).

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