[llvm] [LLVM][DebugInfo]Allow ExtraData field to be a node reference (PR #165023)

[Alexander Yermolovich via llvm-commits]

ayermolo wrote:

This is used to support numba-cuda debugging effort:
https://github.com/NVIDIA/numba-cuda/pull/544/files

```
# Polymorphic debug info with DW_TAG_variant
                            # extraData depends on llvmlite version
                            if config.CUDA_DEBUG_POLY_USE_TYPED_CONST:
                                metadata_dict["extraData"] = ir.IntType(8)(
                                    index
                                )
                            else:
                                # Use metadata node reference
                                metadata_dict["extraData"] = m.add_metadata(
                                    [ir.IntType(8)(index)]
                                ) 
```

https://github.com/llvm/llvm-project/pull/165023