[llvm] [DirectX] Add `llvm.dx.resource.getdimensions` intrinsic and lowering to DXIL (PR #161753)

[Justin Bogner via llvm-commits]

https://github.com/bogner commented:

Generally looks good, but I'd like some extra validation since this op has a rather complicated contract, and we should also add a blurb to llvm's [DXILResources.rst](https://github.com/llvm/llvm-project/blob/main/llvm/docs/DirectX/DXILResources.rst#resource-operations) about the intrinsic and the way it's validated.

https://github.com/llvm/llvm-project/pull/161753