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

[Chris B via cfe-commits]

llvm-beanz wrote:

> You can use `type <{ float, <2 x float>}>` if you need the tightly-packed layout.

I think we need to figure out how we're going to lower this too. Loading from this memory space requires loading 128 bytes at a time, and we need to slice it down to just the parts of the structure we need.

@bogner is working on the lowering logic, so I'd like to make sure what we capture at the IR level is appropriate for what he needs in the DirectX backend.

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