[HLSL][SPIRV] Handling padding in cbuffers in the spir-v backend.

[s-perron]

The latest plane is for cbuffers to have explicit padding when needed. However, the backend should not emit those members. This task has a few parts:

• Revert the changes for the spirv.Layout type when it is no longer used in the FE.
• Do not emit the padding members in the llvm-ir in the spir-v type.
• Identify {{T, padding}[n-1], T}} as an array T[n] in spir-v.