Support Result<T, E> across FFI when niche optimization can be used (v2) This PR is identical to #122253, which was approved and merged but then removed from master by a force-push due to a [CI bug](https://rust-lang.zulipchat.com/#narrow/stream/242791-t-infra/topic/ci.20broken.3F). r? ghost Original PR description: --- Allow allow enums like `Result<T, E>` to be used across FFI if the T/E can be niche optimized and the non-niche-optimized type is FFI safe. Implementation of https://github.com/rust-lang/rfcs/pull/3391 Tracking issue: https://github.com/rust-lang/rust/issues/110503 Additional ABI and codegen tests were added in https://github.com/rust-lang/rust/pull/115372