Initial check-in for mock icd which is being built to allow validation
testing without the need for an actual Vulkan device.

ICD is currently building and passing 324/332 tests. It creates ptr
handles for dispatchable objects and unique id handles for
non-dispatchable objects. It currently has some hard-coded values for
various Get* device-query functions in order to allow forward progress.
The long-term intention is to allow the device configuration to be set
by the test itself.

See the ICD README.md file for more info.