cmState::Snapshot: Provide accessor for the cmState.

Source/cmState.cxx

@@ -787,3 +787,8 @@ cmState::Snapshot cmState::Snapshot::GetBuildsystemDirectoryParent() const
 
   return snapshot;
 }
+
+cmState* cmState::Snapshot::GetState() const
+{
+  return this->State;
+}

Source/cmState.h

@@ -47,6 +47,8 @@ public:
     bool IsValid() const;
     Snapshot GetBuildsystemDirectoryParent() const;
 
+    cmState* GetState() const;
+
   private:
     void ComputeRelativePathTopSource();
     void ComputeRelativePathTopBinary();