[clang] [HLSL] Constant buffers codegen (PR #124886)

[Justin Bogner via cfe-commits]

https://github.com/bogner commented:

This is looking pretty good, though I have some misgivings about `CommonHLSLTargetCodeGenInfo` and think it might be better to introduce a `HLSLLayoutBuilder` that can be used by targets as necessary instead. If you take that suggestion a couple of my comments become moot.

https://github.com/llvm/llvm-project/pull/124886