The new clang_getCompletionCursor function returns the CXCursor stored
in AllocatedCXCodeCompleteResults for a completion result identified
by the index, i.e. offset into CXCodeCompleteResults.Results.
If the result cannot be represented by a CXCursor, a null cursor is
returned. The same happens when the index is out of bounds.
TODO: Build a CXCursor for Macros that have a IdentifierInfo in the
CodeCompletionResult. I do not yet know how to get my hands
on a MacroDefinitionRecord for that identifier, which I can
then pass to cxcursor::MakeCXCursor.
note that this cannot be const, as Result.CreateCodeCompletionString is not const...