Add frozen_string_literal from ruby/ruby@3e92b63

Using the following script:

```bash
grep -L frozen_string_literal lib/**/*.rb test/*.rb|xargs ruby -Ka -e'ARGV.each{|fn|puts
fn;open(fn,"r+"){|f|s=f.read.sub(/\A(#!.*\n)?(#.*coding.*\n)?/,"\\&# frozen_string_literal: false\n");f.rewind;f.write s}}'
```

Author: Zachary Scott

lib/rdoc/alias.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: false
 ##
 # Represent an alias, which is an old_name/new_name pair associated with a
 # particular context

lib/rdoc/anon_class.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: false
 ##
 # An anonymous class like:
 #

lib/rdoc/any_method.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: false
 ##
 # AnyMethod is the base class for objects representing methods
 

lib/rdoc/attr.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: false
 ##
 # An attribute created by \#attr, \#attr_reader, \#attr_writer or
 # \#attr_accessor

lib/rdoc/class_module.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: false
 ##
 # ClassModule is the base class for objects representing either a class or a
 # module.

lib/rdoc/code_object.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: false
 ##
 # Base class for the RDoc code tree.
 #

lib/rdoc/code_objects.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: false
 # This file was used to load all the RDoc::CodeObject subclasses at once.  Now
 # autoload handles this.
 

lib/rdoc/comment.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: false
 ##
 # A comment holds the text comment for a RDoc::CodeObject and provides a
 # unified way of cleaning it up and parsing it into an RDoc::Markup::Document.

lib/rdoc/constant.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: false
 ##
 # A constant
 

lib/rdoc/context.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: false
 require 'cgi'
 
 ##