Implement "Go to Implementation" to find all the derived classes

Any sort of class inheritance info/viewing is unlikely to be available any time soon -- I don't think even Visual Studio has this available. Maybe you could parse the find all references result to check for references of classes that involve inheritance, but that would only work if the base class reference were on the same line as the sub class reference.

VSCode has implemented 'go to implementation' from version 1.35. If I can get the implementation locations of symbol, I think it's ok.

Originally posted by @Lennon925 in https://github.com/microsoft/vscode-cpptools/issues/3886#issuecomment-512705520

まず、既存の find-all-references パスと、それがシンボルの実装箇所をどのように公開しているかを調べます。基準点として VSCode 1.35 の「Go to Implementation」の動作を使用します。完了の条件は、拡張機能がシンボルについて派生クラスの実装をすべて見つけられることです。