[mlir] Add CastInfo for mlir classes subclassing from PointerUnion

This is required to use the function variants of cast/isa/dyn_cast/etc
on them.

Context:
- https://mlir.llvm.org/deprecation/ at "Use the free function variants for dyn_cast/cast/isa/…"
- Original discussion at https://discourse.llvm.org/t/preferred-casting-style-going-forward/68443