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

```

2 files changed, 2 insertions, 2 deletions

diff --git a/actionpack/lib/action_dispatch/routing/mapper.rb b/actionpack/lib/action_dispatch/routing/mapper.rb
index 07e3be4db8..3f7cf0950d 100644
--- a/actionpack/lib/action_dispatch/routing/mapper.rb
+++ b/actionpack/lib/action_dispatch/routing/mapper.rb
@@ -308,7 +308,7 @@ module ActionDispatch
           def check_controller_and_action(path_params, controller, action)
             hash = check_part(:controller, controller, path_params, {}) do |part|
               translate_controller(part) {
-                message = "'#{part}' is not a supported controller name. This can lead to potential routing problems.".dup
+                message = +"'#{part}' is not a supported controller name. This can lead to potential routing problems."
                 message << " See https://guides.rubyonrails.org/routing.html#specifying-a-controller-to-use"
 
                 raise ArgumentError, message
diff --git a/actionpack/lib/action_dispatch/routing/route_set.rb b/actionpack/lib/action_dispatch/routing/route_set.rb
index 07d3a41173..acce8a7ef3 100644
--- a/actionpack/lib/action_dispatch/routing/route_set.rb
+++ b/actionpack/lib/action_dispatch/routing/route_set.rb
@@ -245,7 +245,7 @@ module ActionDispatch
                   missing_keys << missing_key
                 }
                 constraints = Hash[@route.requirements.merge(params).sort_by { |k, v| k.to_s }]
-                message = "No route matches #{constraints.inspect}".dup
+                message = +"No route matches #{constraints.inspect}"
                 message << ", missing required keys: #{missing_keys.sort.inspect}"
 
                 raise ActionController::UrlGenerationError, message