[swift2objc] Visitor infra

[liamappelbe]

This is the same infra as ffigen uses: #1638.

• Copied the AstNode classes etc from ffigen
• Made all the AST classes extends AstNode, and implemented the visit and visitChildren methods.
  • This meant that BuiltInDeclaration had to be converted from an enum to a class
• Rewrote DependencyVisitor using the Visitation class

Ideally we'd share the AstNode, Visitor, and Visitation classes, but we don't really have a way of sharing internal code between native packages yet.

Part of #1358

[coveralls]

coverage: 87.4% (-0.3%) from 87.691%
when pulling 29cd1bc on swiftvisit2
into 3832e4b on main.

[liamappelbe]

@dcharkes @HosseinYousefi This is ready to go. PTAL

[dcharkes]

The documentation could be improved.

If I understand it correctly, the visitor does the traversal, and the visitation contains the methods on what to modify for each node type.

"Wrapper" doesn't say what it's responsibility is and isn't.

I'd maybe call the the class that determines the traversal order the Traverser. Or TraversalStrategy (terminology taken from https://strategoxt.org/, which also explicitly separates the function applied to a node on visit, and the traversal strategy of traversing the whole tree).

[liamappelbe]

Done. I prefer to have the names of tightly coupled classes like this be closely related though. Visitor and Visitation makes them sound more related than Traverser and Visitation.

[HosseinYousefi]

Suggested change

	/// the heirarchy extends [AstNode]:
	/// the hierarchy extends [AstNode]:

[liamappelbe]

Done

[HosseinYousefi]

Suggested change

	/// heirarchy of all AstNodes. Eg `visitChild` should default to calling
	/// hierarchy of all AstNodes. Eg `visitChild` should default to calling

:P

[liamappelbe]

Done

[HosseinYousefi]

nit: Could also be node.visitChildren instead of ..

[liamappelbe]

Oops. A leftover from when this was a transformer that returned nodes.

[github-actions]

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