The boxed layout of a struct always has its data at +8, as evidenced by
`Object::UnBox`. This means that `StackAllocatedBox<T>` should have
`Pack = 1`, otherwise this may not be the case. In the test failure we
had a `StackAllocatedBox<Int128>` which had its `_value` field at offset
16. After object stack allocation this meant that we were saving data in
padding of the structure, which promotion does not guarantee to
preserve.

Fix #106947