Add MVKPhysicalDeviceMetalFeatures::mslVersionEnum and set from Metal feature sets.
Derive MVKPhysicalDeviceMetalFeatures:: mslVersion from mslVersionEnum.
MVKDevice::getMTLCompileOptions() sets MTLCompileOptions::languageVersion
from MVKPhysicalDeviceMetalFeatures::mslVersionEnum.
MVKAssert logs error even when assertions disabled.
Update VK_MVK_MOLTENVK_SPEC_VERSION to 18.