Fix the defined? value for frozen string literals.

Author: nirvdrum

src/main/java/org/truffleruby/parser/BodyTranslator.java

@@ -2886,7 +2886,7 @@ public RubyNode visitStrNode(StrParseNode node) {
         if (node.isFrozen() && !inCore()) {
             final DynamicObject frozenString = context.getFrozenStringLiteral(rope);
 
-            ret = new DefinedWrapperNode(context.getCoreStrings().METHOD,
+            ret = new DefinedWrapperNode(context.getCoreStrings().EXPRESSION,
                     new ObjectLiteralNode(frozenString));
         } else {
             ret = new StringLiteralNode(rope);