WIP AST node visitor

[duckdoom5]

Draft implementation for using clang's AST node visitor. (Not fully functional nor complete)

The current implementation is based on clang's JSONNodeDumper, but the plan is to replace all the stringified properties with setting the raw data on nodes.

So this works like:

• <clang does node iteration>
• <clang calls Visit(const clang::<BaseType>*)> (eg. void ASTNodeVisitor::Visit(const Stmt* S))
• We call InnerStmtVisitor::Visit()
• <clang does inner node iteration>
• <clang calls Visit<MostDerivedType>>
• We override this visitor call (See visitor implementation starting here)
• We check if the node has already been processed
• If not add a new node, and call Convert<MostDerivedType>(clang::<NodeType>, AST::<NodeType>)
• Convert... will then call Convert...Impl on itself and it's base types

• See the conversion implementations, starting here
  • Also not that the Convert...Impl is the exact same code as is generated into ParseExpr/Stmt.h, in other words, with a little bit of work it should be possible to apply this for expr/stmt parsing right now