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

[Justin Bogner via llvm-commits]

bogner wrote:

PR title needs update now that we've gone with `getdimesions.x` rather than `getdimensions.buffer`

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