Function fnptr_as_fnsig 

fn fnptr_as_fnsig<'tcx>(
    fn_sig_tys: &'tcx Binder<'tcx, FnSigTys<TyCtxt<'tcx>>>,
    header: &'tcx FnHeader<TyCtxt<'tcx>>,
) -> Binder<'tcx, FnSig<'tcx>>