DfaValue made abstract (was actually abstract by spirit; constructor is protected): this also fixes false-positives of "Call to default toString" inspection on DfaValues

java/java-analysis-impl/src/com/intellij/codeInspection/dataFlow/value/DfaValue.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2000-2009 JetBrains s.r.o.
+ * Copyright 2000-2017 JetBrains s.r.o.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -15,7 +15,7 @@
  */
 package com.intellij.codeInspection.dataFlow.value;
 
-public class DfaValue {
+public abstract class DfaValue {
   private final int myID;
   protected final DfaValueFactory myFactory;