Improve FrozenStringLiteralComment autocorrection.

By inserting before the whole document, the insertion will happen before all corrections
except possibly those that also act on the whole document.

lib/rubocop/cop/style/frozen_string_literal_comment.rb

@@ -214,7 +214,7 @@ def insert_comment(corrector)
           if comment
             corrector.insert_after(line_range(comment.line), following_comment)
           else
-            corrector.insert_before(line_range(1), preceding_comment)
+            corrector.insert_before(processed_source.buffer.source_range, preceding_comment)
           end
         end
 

spec/rubocop/cli/cli_disable_uncorrectable_spec.rb

@@ -19,6 +19,7 @@
       YAML
       create_file('example.rb', <<~RUBY)
         ip('1.2.3.4')
+        # last line
       RUBY
     end
     let(:max_length) { 46 }
@@ -59,6 +60,7 @@
           # frozen_string_literal: true
 
           ip('1.2.3.4') # rubocop:todo Style/IpAddresses
+          # last line
         RUBY
       end
 
@@ -171,16 +173,17 @@ def choose_move(who_to_move) # rubocop:todo Metrics/CyclomaticComplexity
           == example.rb ==
           C:  1:  1: [Corrected] Style/FrozenStringLiteralComment: Missing frozen string literal comment.
           C:  1:  4: [Todo] Style/IpAddresses: Do not hardcode IP addresses.
-          C:  3:  1: [Corrected] Layout/EmptyLineAfterMagicComment: Add an empty line after magic comments.
+          C:  2:  1: [Corrected] Layout/EmptyLineAfterMagicComment: Add an empty line after magic comments.
 
           1 file inspected, 3 offenses detected, 3 offenses corrected
         OUTPUT
         expect(IO.read('example.rb')).to eq(<<~RUBY)
-          # rubocop:todo Style/IpAddresses
           # frozen_string_literal: true
 
+          # rubocop:todo Style/IpAddresses
           ip('1.2.3.4')
           # rubocop:enable Style/IpAddresses
+          # last line
         RUBY
       end
     end