[DirectedGraph]: Add setTargetNode member function

[etiotto]

Summary

Add the setTargetNode member function to class DGEdge.

[bmahjour]

LGTM, but I'll let another reviewer give the green light :)

[etiotto]

ping

[etiotto]

ping

[Meinersbur]

LGTM

llvm/include/llvm/ADT/DirectedGraph.h

public:
 
   /// Retrieve the target node this edge connects to.
   const NodeType &getTargetNode() const { return TargetNode; }
   NodeType &getTargetNode() {
     return const_cast<NodeType &>(
         static_cast<const DGEdge<NodeType, EdgeType> &>(*this).getTargetNode());
   }
 
+  /// Set the target node this edge connects to.
+  void setTargetNode(const NodeType &N) { TargetNode = N; }
+
 protected:
   // As the default implementation use address comparison for equality.
   bool isEqualTo(const EdgeType &E) const { return this == &E; }
 
   // Cast the 'this' pointer to the derived type and return a reference.
   EdgeType &getDerived() { return *static_cast<EdgeType *>(this); }
   const EdgeType &getDerived() const {
     return *static_cast<const EdgeType *>(this);