[ruby/irb] Remove remaining `frozen_string_literal: false` in lib/

(https://github.com/ruby/irb/pull/883)

https://github.com/ruby/irb/commit/4bfdb23ae6

lib/irb.rb

@@ -1,4 +1,4 @@
-# frozen_string_literal: false
+# frozen_string_literal: true
 #
 #   irb.rb - irb main module
 #       by Keiju ISHITSUKA(keiju@ruby-lang.org)
@@ -903,8 +903,8 @@ module IRB
     # parsed as a :method_add_arg and the output won't be suppressed
 
     PROMPT_MAIN_TRUNCATE_LENGTH = 32
-    PROMPT_MAIN_TRUNCATE_OMISSION = '...'.freeze
-    CONTROL_CHARACTERS_PATTERN = "\x00-\x1F".freeze
+    PROMPT_MAIN_TRUNCATE_OMISSION = '...'
+    CONTROL_CHARACTERS_PATTERN = "\x00-\x1F"
 
     # Returns the current context of this irb session
     attr_reader :context
@@ -1056,7 +1056,7 @@ module IRB
       return read_input(prompt) if @context.io.respond_to?(:check_termination)
 
       # nomultiline
-      code = ''
+      code = +''
       line_offset = 0
       loop do
         line = read_input(prompt)

lib/irb/inspector.rb

@@ -1,4 +1,4 @@
-# frozen_string_literal: false
+# frozen_string_literal: true
 #
 #   irb/inspector.rb - inspect methods
 #   	by Keiju ISHITSUKA(keiju@ruby-lang.org)
@@ -113,7 +113,7 @@ module IRB # :nodoc:
     Color.colorize_code(v.inspect, colorable: Color.colorable? && Color.inspect_colorable?(v))
   }
   Inspector.def_inspector([true, :pp, :pretty_inspect], proc{require_relative "color_printer"}){|v|
-    IRB::ColorPrinter.pp(v, '').chomp
+    IRB::ColorPrinter.pp(v, +'').chomp
   }
   Inspector.def_inspector([:yaml, :YAML], proc{require "yaml"}){|v|
     begin