Code quality improvements + add dedicated DeadRefTypes test

java/ql/lib/semmle/code/java/Element.qll

@@ -41,6 +41,9 @@ class Element extends @element, Top {
 
   /** Cast this element to a `Documentable`. */
   Documentable getDoc() { result = this }
+
+  /** Holds if this is an auxiliary program element generated by the compiler. */
+  predicate isCompilerGenerated() { compiler_generated(this, _) }
 }
 
 /**