Dup possibly frozen string in to_string_literal helper

Currently, there are exactly three instances where the receiver-mutating
`String#force_encoding` method is used. In all cases except for
`to_string_literal`, a `String#dup` call is included to ensure compatibility
with frozen strings. I couldn't identify any counterexamples in rubocop core that would result
in a runtime error, but there is at least one instance in `rubocop-performance` [1]
where a frozen string might be passed. To ensure everything works correctly,
we need to add a `#dup` call in the implementation of `to_string_literal`.

For additional context, it's worth noting that the Prism parser might start
freezing AST leaf nodes in the future [2],
and whitequark's parser has some unpredictability in this regard [3].

[1] rubocop/rubocop-performance#480
[2] ruby/prism#3309
[3] rubocop/rubocop-ast#342

changelog/fix_dup_possibly_frozen_string_in_to_string_literal.md

@@ -0,0 +1 @@
+* [#13605](/~https://github.com/rubocop/rubocop/pull/13605): Fix `RuboCop::Cop::Util.to_string_literal` to work correctly with frozen strings. ([@viralpraxis][])

lib/rubocop/cop/util.rb

@@ -204,8 +204,7 @@ def parse_regexp(text)
       private
 
       def compatible_external_encoding_for?(src)
-        src = src.dup if RUBY_ENGINE == 'jruby'
-        src.force_encoding(Encoding.default_external).valid_encoding?
+        src.dup.force_encoding(Encoding.default_external).valid_encoding?
       end
 
       def include_or_equal?(source, target)

spec/rubocop/cop/util_spec.rb

@@ -86,4 +86,14 @@ def some_method
       expect(described_class.parse_regexp('+')).to be_nil
     end
   end
+
+  describe '#to_string_literal' do
+    it 'returns literal for normal string' do
+      expect(TestUtil.new.send(:to_string_literal, 'foo')).to eq("'foo'")
+    end
+
+    it 'returns literal for string which requires escaping' do
+      expect(TestUtil.new.send(:to_string_literal, 'foo\'')).to eq('"foo\'"')
+    end
+  end
 end