Function gen_fn 

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