Primitive representations don't allow 128-bit types

`repr(i128)` and `repr(u128)` are currently not allowed on enums. However the [Primitive representations](https://github.com/rust-lang/reference/blob/master/src/type-layout.md#primitive-representations) section does not explain that.
cc https://github.com/rust-lang/rust/issues/56071