Post.fast_count: fix incorrect counts for aliased tags.

Revert optimization from a6163258b. Turns out that we have to resolve
aliases in fast_count, otherwise for aliased tags we'll return an empty
count.

Fixes #4156.

test/unit/post_test.rb

@@ -2550,6 +2550,12 @@ class PostTest < ActiveSupport::TestCase
         end
       end
 
+      context "an aliased tag" do
+        should "return the count of the consequent tag" do
+          assert_equal(Post.fast_count("aaa"), Post.fast_count("alias"))
+        end
+      end
+
       context "a single metatag" do
         should "return the correct cached count" do
           FactoryBot.build(:tag, name: "score:42", post_count: -100).save(validate: false)

test/unit/related_tag_calculator_test.rb

@@ -61,6 +61,14 @@ class RelatedTagCalculatorTest < ActiveSupport::TestCase
         assert_equal(%w[1girl 1boy solo], RelatedTagCalculator.similar_tags_for_search("rating:q").pluck(:name))
         assert_equal(%w[solo 1girl], RelatedTagCalculator.similar_tags_for_search("solo").pluck(:name))
       end
+
+      should "calculate the similar tags for an aliased tag" do
+        create(:tag_alias, antecedent_name: "rabbit", consequent_name: "bunny")
+        create(:post, tag_string: "bunny dog")
+        create(:post, tag_string: "bunny cat")
+
+        assert_equal(%w[bunny cat dog], RelatedTagCalculator.similar_tags_for_search("rabbit").pluck(:name))
+      end
     end
   end
 end