Prevent ABI changes affect EnzymeAD
This PR handles ABI changes for autodiff input arguments to improve Enzyme compatibility. Fundamentally this adjusts activities when a function argument is lowered as an `ScalarPair`, so there's no mismatch between diff activities and args. Also removes activities corresponding to ZSTs.
fixes: https://github.com/rust-lang/rust/issues/144025
r? `@ZuseZ4`