[HLSL][SPIRV] Add pass to rewrite CBuffer accesses

[s-perron]

A new pass will be added to the SPIR-V backend to implement Lowering to buffer load intrinsics. This pass has been implemented for DXIL, but we want a different codegen for SPIR-V.

• Update the SPIR-V design doc to mention the pass and how it will work.
• Update the CBuffer design doc to mention that SPIR-V will have its own implementation.
• Implement the pass in the backend.