Avoid string interning when cloning StringNodes.

This shows up as the main cost of cloning AST trees, but it isn't necessary as we already know it is intern'd.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=173996784

src/com/google/javascript/rhino/Node.java

@@ -317,6 +317,11 @@ private static final class StringNode extends Node {
 
     private static final long serialVersionUID = 1L;
 
+    // Only for cloneNode
+    private StringNode(Token token) {
+      super(token);
+    }
+
     StringNode(Token token, String str) {
       super(token);
       setString(str);
@@ -379,7 +384,9 @@ public void setQuotedString() {
 
     @Override
     public StringNode cloneNode(boolean cloneTypeExprs) {
-      return copyNodeFields(new StringNode(token, str), cloneTypeExprs);
+      StringNode clone = new StringNode(token);
+      clone.str = str;
+      return copyNodeFields(clone, cloneTypeExprs);
     }
 
     @GwtIncompatible("ObjectInputStream")