[llvm] [DXIL] Define and generate `DXILAttribute` and `DXILProperty` (PR #117072)

[Finn Plummer via llvm-commits]

inbelic wrote:

Rebased to resolve existing conflicts. I also added two more commits to this pr regarding formatting which will let us also resolve https://github.com/llvm/llvm-project/issues/115912. In hindsight, this pr already completed most of the work tasks for both issues, so it seems better to just have one pr rather than going back and detangling the tasks into 2 prs.

https://github.com/llvm/llvm-project/pull/117072