[clang] [HLSL] support packoffset in clang codeGen (PR #91999)

[Chris B via cfe-commits]

https://github.com/llvm-beanz commented:

I'm a little concerned about the strategy here. LLVM generally doesn't explicitly capture padding it relies on the data layout rules to capture that.

If this is the approach we're going with you can't put any vectors or matrices into the cbuffer structure type and we need to account for all the padding bytes explicitly.

https://github.com/llvm/llvm-project/pull/91999