make sure getIdentifier returns an interned string in ReadVariadicComponentNode

com.oracle.truffle.r.nodes/src/com/oracle/truffle/r/nodes/access/ReadVariadicComponentNode.java

@@ -102,7 +102,7 @@ public class ReadVariadicComponentNode extends RSourceSectionNode implements RSy
 
     @Override
     public String getIdentifier() {
-        return getPrintForm();
+        return getPrintForm().intern();
     }
 
     @Override

com.oracle.truffle.r.runtime/src/com/oracle/truffle/r/runtime/nodes/RSyntaxLookup.java

@@ -29,6 +29,9 @@ import com.oracle.truffle.api.source.SourceSection;
  */
 public interface RSyntaxLookup extends RSyntaxElement {
 
+    /**
+     * @return The identifier that this lookup represents - this needs to be an interned string.
+     */
     String getIdentifier();
 
     boolean isFunctionLookup();