macOS prior to 10.15.6 does not support native host-coherent image memory

On *macOS* versions prior to *macOS 10.15.6*, native host-coherent image device memory is not available. Because of this, changes made to `VkImage VK_MEMORY_PROPERTY_HOST_COHERENT_BIT` device memory by the CPU or GPU will not be available to the GPU or CPU, respectively, until the memory is flushed or unmapped by the application.

Applications using `vkMapMemory()` with `VkImage VK_MEMORY_PROPERTY_HOST_COHERENT_BIT`
device memory on *macOS* versions prior to *macOS 10.15.6* must call either `vkUnmapMemory()`, or `vkFlushMappedMemoryRanges()` / `vkInvalidateMappedMemoryRanges()` to ensure memory changes are coherent between the CPU and GPU.

This limitation does **_not_** apply to `VKImage` device memory on *macOS* starting with *macOS 10.15.6*, does not apply to `VKImage` device memory on any version of *iOS* or *tvOS*, and does **_not_** apply to `VKBuffer` device memory on any platform.

Although performance-killing heroics might be possible (such as forcing coherence before and after each affected Vulkan queue submission), given that the restriction expires with current _macOS_ versions, suggest that a portability flag indicating the restriction on earlier _macOS_ versions be added. Perhaps `nativeHostCoherentImageMemory` or `permantentlyMappedCoherentImageMemory`?

No file, test, or entry point is named in the issue. Start by locating the portability flag definitions and existing macOS memory-property handling, then review how platform limitations are represented. Done means the pre-10.15.6 macOS image-memory limitation is represented by a documented portability flag without changing the stated iOS, tvOS, or buffer behavior.

Written by the indexing model from the issue text.