message: common: add big-endian support to Ip6ntohl and Ip6htonl
  This patch resolves the TODO from
  Tests pass on a big-endian machine (IBM Z).
diff: @@ -424,17 +424,19 @@ bool Utility::portInRangeList(const Address::Instance& address, const std::list<
  }
  absl::uint128 Utility::Ip6ntohl(const absl::uint128& address) {
  - // TODO(ccaraman): Support Ip6ntohl for big-endian.
  - static_assert(ABSL_IS_LITTLE_ENDIAN,
  - "Machines using big-endian byte order is not supported for IPv6.");
  +#ifdef ABSL_IS_LITTLE_ENDIAN
  return flipOrder(address);
  +#else
  + return address;
  +#endif
  }
  absl::uint128 Utility::Ip6htonl(const absl::uint128& address) {
  - // TODO(ccaraman): Support Ip6ntohl for big-endian.
  - static_assert(ABSL_IS_LITTLE_ENDIAN,
  - "Machines using big-endian byte order is not supported for IPv6.");
  +#ifdef ABSL_IS_LITTLE_ENDIAN
  return flipOrder(address);
  +#else
  + return address;
  +#endif
  }
  absl::uint128 Utility::flipOrder(const absl::uint128& input) {
  