layers: Retire work early on external signals
layers: Retire work early on external signals

Retire all work up until a signal operation on an external semaphore and
emit a warning about validation that will no longer occur.

Change-Id: I3d46464c2c32e37071d702e20e5197a495d60472

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools