shader_debugprintf and descriptor_indexing examples not working on macOS

[SRSaunders]

Two issues with these two examples on macOS:

1. shader_debugprintf unnecessarily uses VK_API_VERSION_1_1 which for unknown reasons causes a slowdown, and does not enable VKB_ENABLE_PORTABILITY which prevents the sample from running.
2. descriptor_indexing does not enable Metal Argument Buffers which are required for this example. In addition, there is currently another issue with variable descriptor counts on MoltenVK, which needs to be worked around until such time as MVK corrects the issue.

Will submit a PR for this. As part of this PR I will submit a framework enhacement that enables access to the relatively new Vulkan layer settings API (for enabling Vulkan Validation layer features, and MoltenVK's layer features).