[HLSL] Support packoffset attribute for cbuffer

[python3kgae]

Documentation for packoffset is at
https://learn.microsoft.com/en-us/windows/win32/direct3dhlsl/dx-graphics-hlsl-variable-packoffset

AC

• A design is created that defines how we will lower this to DXIL
• This is only the parsing support for packoffset, does not include the lowering
• Goes through code-gen to IR metadata
• Tests for the IR representation are created

[davidcook-msft]

Additional tasks needed:

• IR lowering to DXIL task
• @bogner to create additional task

[damyanp]

Waiting on llvm/wg-hlsl#50 before really finalizing what's required here.