XML doc generation support should resovle inheritdoc in additional files

The XML documentation comment support is currently able to handle resolving `` references within the compilation assembly by reusing the logic that is used by Roslyn.

This same functionality isn't currently supported for documentation comments that are resolved directly from XML files. In those cases, we have no semantic model to go off of in order to determine the relationships between types. In those situations, we can come up with something rudimentary that tries to understand inheritance hierarchies based on the documentation ID.