Add SPIRVToMSLConverterOptions::platform to track platform. Default to build platform.
Update default SPIRVToMSLConverterOptions MSL version to 2.1.
MoltenVKShaderConverter test MSL compilation use same MSL version as conversion.
Default min perf tracking value to 0.0.
SPIRVToMSLConverter.h reference spirv.hpp via SPIRV-Cross framework.
Update What's New document.