Function differentiate

pub(crate) fn differentiate<'ll, 'tcx>(
    module: &'ll ModuleCodegen<ModuleLlvm>,
    cgcx: &CodegenContext<LlvmCodegenBackend>,
    tcx: TyCtxt<'tcx>,
    diff_items: Vec<AutoDiffItem>,
    config: &ModuleConfig,
) -> Result<(), FatalError>