Function gen_fn

fn gen_fn<'a, 'll, 'tcx>(
    cx: &'a CodegenCx<'ll, 'tcx>,
    name: &str,
    rust_fn_sig: PolyFnSig<'tcx>,
    codegen: &mut dyn FnMut(GenericBuilder<'a, 'll, CodegenCx<'ll, 'tcx>>),
) -> (&'ll Type, &'ll Value)