Don't allocate a bunch of strings in `Relation::Merger`

Since the strings are dynamically computed from a constant, the actual
strings we're creating are a known set. We can compute them ahead of
time, and reduce the number of allocations in that method.

activerecord/lib/active_record/relation/merger.rb

@@ -148,11 +148,15 @@ def merge_single_values
         end
       end
 
+      CLAUSE_METHOD_NAMES = CLAUSE_METHODS.map do |name|
+        ["#{name}_clause", "#{name}_clause="]
+      end
+
       def merge_clauses
-        CLAUSE_METHODS.each do |name|
-          clause = relation.send("#{name}_clause")
-          other_clause = other.send("#{name}_clause")
-          relation.send("#{name}_clause=", clause.merge(other_clause))
+        CLAUSE_METHOD_NAMES.each do |(reader, writer)|
+          clause = relation.send(reader)
+          other_clause = other.send(reader)
+          relation.send(writer, clause.merge(other_clause))
         end
       end
     end