[llvm] [Transforms][DXIL] Wire up a basic DXILUpgrade pass (PR #66275)

[Justin Bogner via llvm-commits]

bogner wrote:

This would be for when the input is DXIL - clang codegen won't generate the dxil-style constructs in the first place.

This is essentially the first step of the discussion here: https://discourse.llvm.org/t/rfc-moving-some-translation-between-dxil-and-standard-llvm-constructs-from-the-directx-backend-to-a-transforms-library/71997

https://github.com/llvm/llvm-project/pull/66275