Function do_fat_lto 

fn do_fat_lto<B: WriteBackendMethods>(
    sess: &Session,
    cgcx: &CodegenContext,
    shared_emitter: SharedEmitter,
    tm_factory: TargetMachineFactoryFn<B>,
    exported_symbols_for_lto: &[String],
    each_linked_rlib_for_lto: &[PathBuf],
    needs_fat_lto: Vec<FatLtoInput<B>>,
) -> CompiledModule