Generate a unique PBXTargetDependency and PBXContainerItemProxy for
every edge in the dependency graph as required by the Xcode format.
Xcode only accidentally loads project files that re-use the same
PBXTargetDependency for every edge leading to a single target.  If one
is removed by hand in the IDE then Xcode may crash due to dangling
references from the others.

Since cmGlobalXCodeGenerator::AddDependTarget is called exactly once for
every edge in the target dependency graph we do not need to keep track
of which edges have been visited to avoid generating duplicates.