Fix Eclipse type-safety warnings in overrides of `ModRef` visitor-making methods (#175)

* Add missing type parameters to overrides of ModRef.makeModVisitor

* Add missing type parameters to overrides of ModRef.makeRefVisitor

This also required slightly generalizing the generic type signature of
the ModRef.makeRefVisitor return type.  Instead of returning
"RefVisitor<T, ExtendedHeapModel>", we now return "RefVisitor<T, ?
extends ExtendedHeapModel>".  We need that extra flexibility because
at least one override of this method returns a RefVisitor whose second
generic type parameter is a subclass of ExtendedHeapModel.

com.ibm.wala.cast.java/src/com/ibm/wala/cast/java/ipa/modref/AstJavaModRef.java

@@ -47,7 +47,7 @@ public class AstJavaModRef<T extends InstanceKey> extends AstModRef<T> {
   }
 
   @Override
-  protected RefVisitor makeRefVisitor(CGNode n, Collection<PointerKey> result, PointerAnalysis<T> pa, ExtendedHeapModel h) {
+  protected RefVisitor<T, ? extends ExtendedHeapModel> makeRefVisitor(CGNode n, Collection<PointerKey> result, PointerAnalysis<T> pa, ExtendedHeapModel h) {
     return new AstJavaRefVisitor<>(n, result, pa, h);
   }
 
@@ -73,7 +73,7 @@ public class AstJavaModRef<T extends InstanceKey> extends AstModRef<T> {
   }
  
   @Override
-  protected ModVisitor makeModVisitor(CGNode n, Collection<PointerKey> result, PointerAnalysis<T> pa, ExtendedHeapModel h, boolean ignoreAllocHeapDefs) {
+  protected ModVisitor<T, ? extends ExtendedHeapModel> makeModVisitor(CGNode n, Collection<PointerKey> result, PointerAnalysis<T> pa, ExtendedHeapModel h, boolean ignoreAllocHeapDefs) {
     return new AstJavaModVisitor<>(n, result, h, pa);
   }
 

com.ibm.wala.cast.js/source/com/ibm/wala/cast/js/ipa/modref/JavaScriptModRef.java

@@ -99,7 +99,7 @@ public class JavaScriptModRef<T extends InstanceKey> extends AstModRef<T> {
   }
 
   @Override
-  protected RefVisitor makeRefVisitor(CGNode n, Collection<PointerKey> result, PointerAnalysis<T> pa, ExtendedHeapModel h) {
+  protected RefVisitor<T, ? extends ExtendedHeapModel> makeRefVisitor(CGNode n, Collection<PointerKey> result, PointerAnalysis<T> pa, ExtendedHeapModel h) {
     return new JavaScriptRefVisitor<>(n, result, pa, h);
   }
 
@@ -168,7 +168,7 @@ public class JavaScriptModRef<T extends InstanceKey> extends AstModRef<T> {
   }
 
   @Override
-  protected ModVisitor makeModVisitor(CGNode n, Collection<PointerKey> result, PointerAnalysis<T> pa, ExtendedHeapModel h, boolean ignoreAllocHeapDefs) {
+  protected ModVisitor<T, ? extends ExtendedHeapModel> makeModVisitor(CGNode n, Collection<PointerKey> result, PointerAnalysis<T> pa, ExtendedHeapModel h, boolean ignoreAllocHeapDefs) {
     return new JavaScriptModVisitor<>(n, result, h, pa);
   }
 }

com.ibm.wala.cast/source/java/com/ibm/wala/cast/ipa/modref/AstModRef.java

@@ -90,7 +90,7 @@ public class AstModRef<T extends InstanceKey> extends ModRef<T> {
   }
 
   @Override
-  protected RefVisitor makeRefVisitor(CGNode n, Collection<PointerKey> result, PointerAnalysis<T> pa, ExtendedHeapModel h) {
+  protected RefVisitor<T, ? extends ExtendedHeapModel> makeRefVisitor(CGNode n, Collection<PointerKey> result, PointerAnalysis<T> pa, ExtendedHeapModel h) {
     return new AstRefVisitor<>(n, result, pa, (AstHeapModel)h);
   }
 
@@ -150,7 +150,7 @@ public class AstModRef<T extends InstanceKey> extends ModRef<T> {
   }
 
   @Override
-  protected ModVisitor makeModVisitor(CGNode n, Collection<PointerKey> result, PointerAnalysis<T> pa, ExtendedHeapModel h, boolean ignoreAllocHeapDefs) {
+  protected ModVisitor<T, ? extends ExtendedHeapModel> makeModVisitor(CGNode n, Collection<PointerKey> result, PointerAnalysis<T> pa, ExtendedHeapModel h, boolean ignoreAllocHeapDefs) {
     return new AstModVisitor<>(n, result, (AstHeapModel)h, pa);
   }
 

com.ibm.wala.core/src/com/ibm/wala/ipa/modref/ModRef.java

@@ -342,7 +342,7 @@ public class ModRef<T extends InstanceKey> {
     return makeModVisitor(n, result, pa, h, false);
   }
 
-  protected ModVisitor<T,? extends ExtendedHeapModel> makeModVisitor(CGNode n, Collection<PointerKey> result, PointerAnalysis<T> pa, ExtendedHeapModel h,
+  protected ModVisitor<T, ? extends ExtendedHeapModel> makeModVisitor(CGNode n, Collection<PointerKey> result, PointerAnalysis<T> pa, ExtendedHeapModel h,
       boolean ignoreAllocHeapDefs) {
     return new ModVisitor<>(n, result, h, pa, ignoreAllocHeapDefs);
   }
@@ -368,8 +368,8 @@ public class ModRef<T extends InstanceKey> {
     return hexcl == null ? result : hexcl.filter(result);
   }
 
-  protected RefVisitor<T, ExtendedHeapModel> makeRefVisitor(CGNode n, Collection<PointerKey> result, PointerAnalysis<T> pa, ExtendedHeapModel h) {
-    return new RefVisitor<T, ExtendedHeapModel>(n, result, pa, h);
+  protected RefVisitor<T, ? extends ExtendedHeapModel> makeRefVisitor(CGNode n, Collection<PointerKey> result, PointerAnalysis<T> pa, ExtendedHeapModel h) {
+    return new RefVisitor<>(n, result, pa, h);
   }
 
   /**