Ignore ZST offsets when deciding whether to use Scalar/ScalarPair layout This is important because Scalar/ScalarPair layout previously would not be used if any ZST had nonzero offset. For example, before this change, only `((), u128)` would be laid out like `u128`, not `(u128, ())`. Fixes #63244
Please read the rustc-dev-guide chapter on Backend Agnostic Codegen.