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

This pass will upgrade DXIL-style llvm constructs (which are mostly
metadata) into the representations we use in LLVM for the same concepts.

For now we just strip the valver metadata, which we don't need. Later
changes will make this pass more useful, and then we should be able to
wire it into clang and possibly the DirectX backend's AsmParser.