Add supertype to swift nodes

pmd-swift/src/main/antlr4/net/sourceforge/pmd/lang/swift/ast/Swift.g4

@@ -54,7 +54,7 @@ import net.sourceforge.pmd.lang.ast.impl.antlr4.*;
 }
 
 options {
-    contextSuperClass = AntlrBaseNode;
+    contextSuperClass = SwiftNode;
     superClass = PmdAntlrParserBase;
 }
 

pmd-swift/src/main/java/net/sourceforge/pmd/lang/swift/ast/SwiftNode.java

@@ -0,0 +1,29 @@
+/*
+ * BSD-style license; for more info see http://pmd.sourceforge.net/license.html
+ */
+
+package net.sourceforge.pmd.lang.swift.ast;
+
+import org.antlr.v4.runtime.ParserRuleContext;
+import org.antlr.v4.runtime.tree.ParseTree;
+
+import net.sourceforge.pmd.lang.ast.impl.antlr4.AntlrBaseNode;
+
+/**
+ * Supertype of all swift nodes.
+ */
+public abstract class SwiftNode extends AntlrBaseNode<SwiftNode> {
+
+    protected SwiftNode() {
+        super();
+    }
+
+    protected SwiftNode(final ParserRuleContext parent, final int invokingStateNumber) {
+        super(parent, invokingStateNumber);
+    }
+
+    @Override
+    protected SwiftNode cast(ParseTree o) {
+        return (SwiftNode) o;
+    }
+}