Experimental: Support SPV_KHR_untyped_pointers

[alan-baker]

SPV_KHR_untyped_pointers provides similar functionality in SPIR-V to opaque pointers in LLVM. There is currently no corresponding Vulkan extension, but work in clspv can begin in the interim.

Necessary changes:

• Supported SPIR-V storage classes:
  • StorageBuffer/PhysicalStorageBuffer
  • Uniform
  • Workgroup
• Implicit pointer casts in affected storage classes can be left in place
• SPIRVProducer needs to layout Workgroup/local data types
  • Support SPV_KHR_workgroup_memory_explicit_layout
  • Add workgroup to 8- and 16-bit storage options
  • Add workgroup scalar layout option
• SPIRVProducer needs to generate new instructions
• Allocate descriptors can disregard types in most cases to share more descriptors
  • Probably the untyped variable should just use a placeholder data type (I expect Vulkan to require some type on the variable)
  • Maybe use uint4 for simplicity to work with every storage class
• Direct resource access might be less necessary
• Others?