[benchmark] DictTest4Legacy: Add dummy hash(into:)

The new definition is the same as the one synthesized by the compiler.

Author: lorentey

benchmark/single-source/DictTest4Legacy.swift

@@ -56,6 +56,10 @@ struct LargeKey: Hashable {
     return hash
   }
 
+  func hash(into hasher: inout Hasher) {
+    hasher.combine(self.hashValue)
+  }
+
   init(_ value: Int) {
     self.i = value
     self.j = 2 * value