cmComputeComponentGraph: Move work out of constructor into Compute() method

The computation of Tarjan's algorithm is an expensive operation which
should not be done in the constructor. Move this work into a dedicated
Compute() method, and call this method explicitly.

Source/cmComputeComponentGraph.h

@@ -31,6 +31,9 @@ public:
   cmComputeComponentGraph(Graph const& input);
   ~cmComputeComponentGraph();
 
+  /** Run the computation.  */
+  void Compute();
+
   /** Get the adjacency list of the component graph.  */
   Graph const& GetComponentGraph() const { return this->ComponentGraph; }
   EdgeList const& GetComponentGraphEdges(int c) const