[201811][swss][utilities] advance sub module head (#2968)

Submodule src/sonic-utilities 6b4d1a0..46b5aa8:
  > [show ip interface] Add support for 'alias' interface naming mode (#486)

Submodule src/sonic-swss 9c4ae18..a637562:
  > Suppress storm detect counter increment for ongoing pfc storm case during a warm reboot (#869)
  > Remove *_LEFT fields to allow PFC watchdog to enter fresh into the (#897)
  > Set LAG mtu value based on kernel netlink msg (#922)
  > [warm restart assist] assume vector values could be reordered (#921)

Signed-off-by: Ying Xie <ying.xie@microsoft.com>