Fix errors in WhyDgml and account for cycles

[jtschuster]

Fixes compiler errors and adds a "visited" parameter to account for cycles in the dependency graph.

[MichalStrehovsky]

The comment on lines 14-15 might need updating if we really want to go with this.

I assume hitting cycles would be pretty common in regular DGML files. I don't know how useful this extra cycle handling is. Sure, now the program will terminate, but answers with (cycle) appended to it are not be very helpful (they don't end up telling the path to the root, which is the entire purpose of the whydgml tool).

One would either need a more sophisticated graph algorithm turning this short tool into something complex, or... just implementing the FirstMarkLogStrategy in ILLinker's DGML writer. My preference would be on the later.

(I don't mind either way, I'm not going to use this with graphs that have cycles.)