cmGeneratorExpressionDAGChecker: Avoid const_cast

Make a member mutable to avoid casting away const of a whole object.

Source/cmGeneratorExpressionDAGChecker.cxx

@@ -68,9 +68,7 @@ void cmGeneratorExpressionDAGChecker::Initialize()
         return;
       }
     }
-    const_cast<cmGeneratorExpressionDAGChecker*>(top)
-      ->Seen[this->Target]
-      .insert(this->Property);
+    top->Seen[this->Target].insert(this->Property);
   }
 }
 

Source/cmGeneratorExpressionDAGChecker.h

@@ -88,7 +88,7 @@ private:
   const cmGeneratorExpressionDAGChecker* const Parent;
   cmGeneratorTarget const* Target;
   const std::string Property;
-  std::map<cmGeneratorTarget const*, std::set<std::string>> Seen;
+  mutable std::map<cmGeneratorTarget const*, std::set<std::string>> Seen;
   const GeneratorExpressionContent* const Content;
   const cmListFileBacktrace Backtrace;
   Result CheckResult;