[openxr] Incorporate openxr-experimental changes into dev

[MarkY-LunarG]

This is a long term issue and depends on several things:

1. Incorporating changes to code gen design back into code gen from the tree
2. Incorporating changes on how handles from multiple Khronos APIs are stored back into the dev tree
3. Incorporating changes to how consumers from multiple Khronos APIs communicate with each other.
4. Stabilizing OpenXR changes