core_validation: add core_validation::global_layer
core_validation: add core_validation::global_layer

Replace cv_global_layers and cv_device_layers with global_layer.  There is
no plan to define more than one layer in the same namespace.

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