[SPIRV] Implement and test `half` type from HLSL

[sudonatalie]

Add tests for the half type compilation from HLSL to SPIR-V with and without the -enable-16bit-types option enabled, and in different Vulkan target environments, which determines whether the Float16 capability requires an extension.

[sudonatalie]

I don't yet have commit access but this issue can be assigned to me with the backend:SPIR-V label