[HLSL] Define RasterizerOrderedBuffer resource

Define HLSL's RasterizerOrderedBuffer resource type through the
external sema source. This doesn't fully work as is, but defining it
allows us to exercise the ROV logic in the DirectX backend from HLSL
rather than having to manually edit metadata, so it's useful for
further testing and development.

Pull Request: https://github.com/llvm/llvm-project/pull/74897