VK_KHR_vulkan_memory_model poorly integrated with the core spec

I am being told there is an internal ticket for this, but all the same documenting some loose ends in the sync spec before I forget:

I am still not exactly clear what features `VK_KHR_vulkan_memory_model` adds. But for sure it seems to contain lot of things that are supposed to be part of even the core 1.0 Vulkan spec. And those things do not yet seem to properly integrated back to that version.

From top of my head these things should be properly specified in the core spec (even retrospectively in some manner):

- "happens-before"\"happens-after", which seems to be used throughout the spec, but not define outside the extension
- domain operations, which gives meaning to synch of GPU–Host transfers
- in-shader barrier sync

Start by reading the VK_KHR_vulkan_memory_model extension alongside the core Vulkan 1.0 synchronization specification in Vulkan-Docs. Determine how “happens-before” and “happens-after,” domain operations, and in-shader barrier synchronization should be defined in the core specification; done means these loose ends are consistently specified there.

Written by the indexing model from the issue text.