Stop passing `whitelisted_items` as an argument to the codegen functions, and use ctx.codegen_items() instead.

This was computed on the fly before and passed as an argument. Now we precompute it in `context.rs`, and just pass `codegen_items()` as an argument.

We should be able to remove that argument altogether and just call `codegen_items` when needed, given we also pass the context pretty much everywhere.