std::is_signed can't be specialized, so this actually lets through non-unsigned types where the types are not language primitives (i.e. it lets absl::int128 through). However, std::numeric_limits can be specialized, and is indeed specialized, so we can use that instead.
PiperOrigin-RevId: 636983590
Change-Id: Ic993518e9cac7c453b08deaf3784b6fba49f15d0