Fix @objcImpl subclass init visibility computation

When writing an @objc subclass of an @objcImplementation class, implicit initializers in the subclass were treated as overriding the *implementation decl*, not the *interface decl*, of the initializer in the superclass. This caused Swift to incorrectly compute the visibility of the superclass initializer and omit an `override` keyword from the module interface when one would definitely be necessary.

Correct this oversight by looking up the interface decl matching the superclass implementation in `collectNonOveriddenSuperclassInits()`.

test/ModuleInterface/objc_implementation.swift

@@ -74,7 +74,7 @@ open class SwiftSubclass: ImplClass {
     print("subclass mainMethod")
   }
 
-  // CHECK-DAG: @objc dynamic public init()
+  // CHECK-DAG: @objc override dynamic public init()
   // CHECK-DAG: @objc deinit
 }
 // CHECK: }