Add SamplerYcbcrConversionMetaData struct to llpc header file
Add SamplerYcbcrConversionMetaData struct to llpc header file

- The struct is used as part of immutable sampler desc.
- The struct should only be used when it is a YCbCr sampler.

googlestadia/llpc,GPUOpen-Drivers/llpc,googlestadia/llpc,GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc,googlestadia/llpc,googlestadia/llpc,GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc,googlestadia/llpc