fix deserialization test failure

Author: Marc Rasi

test/Serialization/derivative_attr.swift

@@ -36,6 +36,10 @@ protocol InstanceMethod : Differentiable {
   func foo(_ x: Self) -> Self
   func bar<T : Differentiable>(_ x: T) -> Self
 }
+extension InstanceMethod {
+  func foo(_ x: Self) -> Self { self }
+  func bar<T : Differentiable>(_ x: T) -> Self { self }
+}
 extension InstanceMethod {
   // CHECK: @derivative(of: foo, wrt: (self, x))
   @derivative(of: foo)