Consider comprehensive design approach for HLSL data alignment

[llvm-beanz]

Is your feature request related to a problem? Please describe.
We have a feature proposal for an alignas attribute, and data alignment has also come up in the context of our ongoing work with HLSL and DXIL vectors.

We should track needing to have some longer-term plans around alignment of data in HLSL.

[devshgraphicsprogramming]

Could scalar-layout be required in the future to introduce less interactions?