Preserve Nodes.InterpolatedStringNode's FROZEN flag when all the parts are String literals

Author: andrykonchin

spec/truffle/parsing/fixtures/strings/concatenated_literal_frozen.yaml

@@ -0,0 +1,16 @@
+subject: "String"
+description: "2 string literals concatenated at parse/translation time / with frozen_string_literal: true"
+notes: >
+  Result string should be frozen and represented as a FrozenStringLiteralNode node
+focused_on_node: "org.truffleruby.language.literal.FrozenStringLiteralNode"
+ruby: |
+  # frozen_string_literal: true
+  "foo" "bar"
+ast: |
+  FrozenStringLiteralNode
+      attributes:
+          definition = "expression" (ASCII-8BIT)
+          flags = 0
+          frozenString = "foobar" (UTF-8)
+          sourceCharIndex = 30
+          sourceLength = 11

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

@@ -2510,7 +2510,8 @@ private static Nodes.StringNode concatStringNodes(Nodes.InterpolatedStringNode n
         int start = parts[0].startOffset;
         var last = ArrayUtils.getLast(parts);
         int length = last.endOffset() - start;
-        return new Nodes.StringNode(NO_FLAGS, concatenated, start, length);
+        short flags = node.isFrozen() ? Nodes.StringFlags.FROZEN : NO_FLAGS;
+        return new Nodes.StringNode(flags, concatenated, start, length);
     }
 
     /** Translate parts of interpolated String, Symbol or Regexp */