Support by the MTLDevice for Metal Tier 2 argument buffers has an
impact on both descriptor indexing and buffer device address support.
Add MVKPhysicalDeviceMetalFeatures::argumentBuffersTier, to track
the MTLDevice argument buffers support tier, set it from
[MTLDevice argumentBuffersSupport], and subsequently enable support for
VK_KHR_buffer_device_address and VK_EXT_buffer_device_address extensions,
and set descriptor indexing resource counts, based on it.
Update documentation requirements.
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