[clang] [llvm] [HLSL] Add ConstantBuffer<T> (PR #195153)

[Steven Perron via cfe-commits]

s-perron wrote:

> I have a PR that is removing implicit constructors for user-defined structs (https://github.com/llvm/llvm-project/pull/194989) which fixes errors in some of the tests you are adding. We should sync-up on which one should go in first.

I'll stack mine on top of yours and fix the tests.

https://github.com/llvm/llvm-project/pull/195153