Make `VALID_DIRECTIONS` to `Set`

```ruby
require "benchmark/ips"
require "set"

array = [:asc, :desc, :ASC, :DESC, "asc", "desc", "ASC", "DESC"]
set   = array.to_set
item  = "DESC"

Benchmark.ips do |x|
  x.report "array" do
    array.include?(item)
  end
  x.report "set" do
    set.include?(item)
  end
end
```

```
% ruby array_vs_set.rb
Warming up --------------------------------------
               array   188.441k i/100ms
                 set   229.531k i/100ms
Calculating -------------------------------------
               array      3.508M (± 9.0%) i/s -     17.525M in   5.043058s
                 set      5.134M (± 7.6%) i/s -     25.707M in   5.038921s
```

1 files changed, 5 insertions, 3 deletions

diff --git a/activerecord/lib/active_record/relation/query_methods.rb b/activerecord/lib/active_record/relation/query_methods.rb
index 76e529f2de..79e65baae5 100644
--- a/activerecord/lib/active_record/relation/query_methods.rb
+++ b/activerecord/lib/active_record/relation/query_methods.rb
@@ -1100,14 +1100,16 @@ module ActiveRecord
       end
 
       VALID_DIRECTIONS = [:asc, :desc, :ASC, :DESC,
-                          "asc", "desc", "ASC", "DESC"] # :nodoc:
+                          "asc", "desc", "ASC", "DESC"].to_set # :nodoc:
 
       def validate_order_args(args)
         args.each do |arg|
           next unless arg.is_a?(Hash)
           arg.each do |_key, value|
-            raise ArgumentError, "Direction \"#{value}\" is invalid. Valid " \
-                                 "directions are: #{VALID_DIRECTIONS.inspect}" unless VALID_DIRECTIONS.include?(value)
+            unless VALID_DIRECTIONS.include?(value)
+              raise ArgumentError,
+                "Direction \"#{value}\" is invalid. Valid directions are: #{VALID_DIRECTIONS.to_a.inspect}"
+            end
           end
         end
       end