[HLSL][SPIRV] Modify the VulkanBuffer implementation to take a struct

[s-perron]

The current implementation of the the spirv.VulkanBuffer type assumes that the element type can be anything, and will wrap it new struct, which will be decorated with the block decoration. This means that the struct decorated with block can have only one member.

This does not work well for CBuffers in HLSL, which can have multiple members in the struct.

• Update the design doc.
• Update the implementation in llvm-project.