Enable `Performance/UnfreezeString` cop

In Ruby 2.3 or later, `String#+@` is available and `+@` is faster than `dup`.

```ruby
# frozen_string_literal: true

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  gem "benchmark-ips"
end

Benchmark.ips do |x|
  x.report('+@') { +"" }
  x.report('dup') { "".dup }
  x.compare!
end
```

```
$ ruby -v benchmark.rb
ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-linux]
Warming up --------------------------------------
                  +@   282.289k i/100ms
                 dup   187.638k i/100ms
Calculating -------------------------------------
                  +@      6.775M (± 3.6%) i/s -     33.875M in   5.006253s
                 dup      3.320M (± 2.2%) i/s -     16.700M in   5.032125s

Comparison:
                  +@:  6775299.3 i/s
                 dup:  3320400.7 i/s - 2.04x  slower

```

1 files changed, 2 insertions, 2 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/mysql/schema_creation.rb b/activerecord/lib/active_record/connection_adapters/mysql/schema_creation.rb
index c9ea653b77..82ed320617 100644
--- a/activerecord/lib/active_record/connection_adapters/mysql/schema_creation.rb
+++ b/activerecord/lib/active_record/connection_adapters/mysql/schema_creation.rb
@@ -17,7 +17,7 @@ module ActiveRecord
           end
 
           def visit_ChangeColumnDefinition(o)
-            change_column_sql = "CHANGE #{quote_column_name(o.name)} #{accept(o.column)}".dup
+            change_column_sql = +"CHANGE #{quote_column_name(o.name)} #{accept(o.column)}"
             add_column_position!(change_column_sql, column_options(o.column))
           end
 
@@ -64,7 +64,7 @@ module ActiveRecord
 
           def index_in_create(table_name, column_name, options)
             index_name, index_type, index_columns, _, _, index_using, comment = @conn.add_index_options(table_name, column_name, options)
-            add_sql_comment!("#{index_type} INDEX #{quote_column_name(index_name)} #{index_using} (#{index_columns})".dup, comment)
+            add_sql_comment!((+"#{index_type} INDEX #{quote_column_name(index_name)} #{index_using} (#{index_columns})"), comment)
           end
       end
     end