Add base class for all AST nodes

[jacob-carlborg]

This is useful to be able to handle all AST nodes the same way, i.e. in an array. Common code for all AST nodes can also be placed here.

[dlang-bot]

Thanks for your pull request and interest in making D better, @jacob-carlborg! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the annotated coverage diff directly on GitHub with CodeCov's browser extension
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

---

If you have addressed all reviews or aren't sure how to proceed, don't hesitate to ping us with a simple comment.

Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + dmd#9408"

[jacob-carlborg]

Blocks #9409.

[WalterBright]

I don't understand. The root object is already RootObject. Why add another layer?

[jacob-carlborg]

I don't understand. The root object is already RootObject. Why add another layer?

I thought RootObject was the base class for all classes, regardless if the class is part of the AST or not. Only classes that are part of the AST should inherit from ASTNode.

As you can see I've added the accept method to ASTNode, if that where to be added to RootObject instead, the dmd.root package would need to import the visitor module. I would have guessed that the dmd.root should not import anything from outside of that package.