Move SimpleDefinedAtom::addReference to DefinedAtom.

Most of the other methods to access Reference's were on DefinedAtom so
this just keeps them all together.

This will be used in a future patch in ObjCPass which needs to add
new references.  The method is virtual because we may add references to
different data structures depending on whether we parsed a macho file or
yaml.

llvm-svn: 259436