CBuffers

[bogner]

• [ ] llvm/wg-hlsl#50
• [ ] llvm/llvm-project#112993
• [ ] llvm/llvm-project#113514
• [ ] llvm/llvm-project#57914
• [ ] llvm/llvm-project#112992
• [ ] Calculate cbuffer layout
• [ ] Update spec with info about cbuffers

[damyanp]

Need to ensure we have issues filed for all of these items.