Switch the ModuleInterface/objc_implementation.swift test to using standard substitutions to properly emit its module interface, and make sure it also verifies.
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()`.
Module interfaces should not include the @objcImplementation attribute, member implementations that are redundant with the ObjC header, or anything that would be invalid in an ordinary extension (e.g. overridden initializers, stored Swift-only properties).
