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/postgresql/schema_statements.rb b/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
index 00da7690a2..fae3ddbad4 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
@@ -686,7 +686,7 @@ module ActiveRecord
           def change_column_sql(table_name, column_name, type, options = {})
             quoted_column_name = quote_column_name(column_name)
             sql_type = type_to_sql(type, options)
-            sql = "ALTER COLUMN #{quoted_column_name} TYPE #{sql_type}".dup
+            sql = +"ALTER COLUMN #{quoted_column_name} TYPE #{sql_type}"
             if options[:collation]
               sql << " COLLATE \"#{options[:collation]}\""
             end
@@ -757,7 +757,7 @@ module ActiveRecord
             scope = quoted_scope(name, type: type)
             scope[:type] ||= "'r','v','m','p','f'" # (r)elation/table, (v)iew, (m)aterialized view, (p)artitioned table, (f)oreign table
 
-            sql = "SELECT c.relname FROM pg_class c LEFT JOIN pg_namespace n ON n.oid = c.relnamespace".dup
+            sql = +"SELECT c.relname FROM pg_class c LEFT JOIN pg_namespace n ON n.oid = c.relnamespace"
             sql << " WHERE n.nspname = #{scope[:schema]}"
             sql << " AND c.relname = #{scope[:name]}" if scope[:name]
             sql << " AND c.relkind IN (#{scope[:type]})"