[clang] [HLSL] Constant buffer layout struct update (PR #124840)

[Helena Kotas via cfe-commits]

hekota wrote:

All of these changes except one are related to layout structs. I will update the PR description to make it clearer and move that one unrelated change about synthetizing initializers to the constant buffer codegen PR.

https://github.com/llvm/llvm-project/pull/124840