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

```

4 files changed, 4 insertions, 4 deletions

diff --git a/railties/lib/rails/commands/dbconsole/dbconsole_command.rb b/railties/lib/rails/commands/dbconsole/dbconsole_command.rb
index 806b7de6d6..0fac7d34a0 100644
--- a/railties/lib/rails/commands/dbconsole/dbconsole_command.rb
+++ b/railties/lib/rails/commands/dbconsole/dbconsole_command.rb
@@ -75,7 +75,7 @@ module Rails
         args += ["-P", "#{config['password']}"] if config["password"]
 
         if config["host"]
-          host_arg = "#{config['host']}".dup
+          host_arg = +"#{config['host']}"
           host_arg << ":#{config['port']}" if config["port"]
           args += ["-S", host_arg]
         end
diff --git a/railties/lib/rails/generators/generated_attribute.rb b/railties/lib/rails/generators/generated_attribute.rb
index f7fd30a5fb..3f20f5a718 100644
--- a/railties/lib/rails/generators/generated_attribute.rb
+++ b/railties/lib/rails/generators/generated_attribute.rb
@@ -153,7 +153,7 @@ module Rails
       end
 
       def inject_options
-        "".dup.tap { |s| options_for_migration.each { |k, v| s << ", #{k}: #{v.inspect}" } }
+        (+"").tap { |s| options_for_migration.each { |k, v| s << ", #{k}: #{v.inspect}" } }
       end
 
       def inject_index_options
diff --git a/railties/lib/rails/info.rb b/railties/lib/rails/info.rb
index d5c9973c6b..3df36efc4c 100644
--- a/railties/lib/rails/info.rb
+++ b/railties/lib/rails/info.rb
@@ -41,7 +41,7 @@ module Rails
       alias inspect to_s
 
       def to_html
-        "<table>".dup.tap do |table|
+        (+"<table>").tap do |table|
           properties.each do |(name, value)|
             table << %(<tr><td class="name">#{CGI.escapeHTML(name.to_s)}</td>)
             formatted_value = if value.kind_of?(Array)
diff --git a/railties/lib/rails/source_annotation_extractor.rb b/railties/lib/rails/source_annotation_extractor.rb
index 2d66a4dc7d..d7170e6282 100644
--- a/railties/lib/rails/source_annotation_extractor.rb
+++ b/railties/lib/rails/source_annotation_extractor.rb
@@ -50,7 +50,7 @@ module Rails
       # If +options+ has a flag <tt>:tag</tt> the tag is shown as in the example above.
       # Otherwise the string contains just line and text.
       def to_s(options = {})
-        s = "[#{line.to_s.rjust(options[:indent])}] ".dup
+        s = +"[#{line.to_s.rjust(options[:indent])}] "
         s << "[#{tag}] " if options[:tag]
         s << text
       end