Issue 7104 - Emit anon class to object file

http://d.puremagic.com/issues/show_bug.cgi?id=7104

This is similar to how https://github.com/D-Programming-Language/dmd/pull/1391 was implemented, except I've moved `objFileDone` up the hierarchy to `DSymbol`.

Since the linker should know how to eliminate unused symbols I wouldn't consider it a problem implementing it this way. But maybe there's a better way?