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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

> The data layout will insert 4-bytes of padding between the float and the vector because the vector needs to be properly aligned.

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

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