Additional changes to Vulkan 1.1 API updates

This commit includes the following:

- Make new/Vulkan 1.1 core commands be optional in codegen.  Before this
  change, the loader crashed when starting with a 1.0 app, and using a 1.0 ICD.
  That's because the loader couldn't find "required" 1.1 entrypoints.  This
  change makes those entrypoints "optional" with a new annotation in the api
  file.

- Changes from internal reviews, including a fix for the algorithm of
  EnumeratePhysicalDeviceGroups.

Test: Run with a 1.0 app with a 1.0 driver and make sure the loader works
Change-Id: I55914b680a1601cb9f3f2cc0257091a0a34ae7f2