Add build environment variable MVK_HIDE_VULKAN_SYMBOLS. to allow MoltenVK
to be built with static Vulkan API symbols hidden, to avoid library linking
conflicts when bound to a Vulkan Loader that also exports identical symbols.
The default value of MVK_HIDE_VULKAN_SYMBOLS is 0,
meaning Vulkan static symbols are exposed by default.
Add MVK_PUBLIC_VULKAN_SYMBOL directive to mark each Vulkan call symbols
for exporting or hiding.
Update the MoltenVK Xcode project to add the MVK_HIDE_VULKAN_SYMBOLS build
setting, and set the ENABLE_TESTABILITY build setting to NO, because it
conflicts with stripping symbols.
Update MoltenVK version to 1.1.7.
5de7f5551c