Test that initializers inherit '@objc' from convenience inits (#23305)

This is weird behavior---there's no 'override' keyword here or
anything! But we rely on it when a convenience initializer written in
Objective-C dispatches to another convenience initializer. Changing
this /will/ break some Apple Objective-C classes that are meant to be
subclassed, unfortunately.

Author: jrose-apple

test/attr/attr_objc_swift4.swift

@@ -1,4 +1,5 @@
 // RUN: %target-swift-frontend -disable-objc-attr-requires-foundation-module -typecheck -verify %s -swift-version 4 -enable-source-import -I %S/Inputs
+// RUN: %target-swift-ide-test -skip-deinit=false -print-ast-typechecked -source-filename %s -prefer-type-repr=false -print-implicit-attrs=true -explode-pattern-binding-decls=true -disable-objc-attr-requires-foundation-module -swift-version 4 -enable-source-import -I %S/Inputs | %FileCheck %s
 // REQUIRES: objc_interop
 
 import Foundation
@@ -30,3 +31,42 @@ class BadInSwift4 {
   @GKInspectable var badGKInspectable: PlainStruct?
   // expected-error@-1{{property cannot be marked @GKInspectable because its type cannot be represented in Objective-C}}
 }
+
+// CHECK-LABEL: class InitsInheritObjCAttrBase
+class InitsInheritObjCAttrBase: NSObject {
+  override init() {}
+  // CHECK: {{^}} @objc convenience init(foo: Int)
+  @objc convenience init(foo: Int) { self.init() }
+} // CHECK: {{^[}]$}}
+
+// CHECK-LABEL: extension InitsInheritObjCAttrBase
+extension InitsInheritObjCAttrBase {
+  // CHECK: {{^}} @objc convenience dynamic init(bar: Int)
+  @objc convenience init(bar: Int) { self.init() }
+} // CHECK: {{^[}]$}}
+
+// CHECK-LABEL: class ConvenienceInitsInheritObjCAttrSub
+class ConvenienceInitsInheritObjCAttrSub: InitsInheritObjCAttrBase {
+  init(somethingElse: ()) { super.init() }
+
+  // CHECK: {{^}} @objc convenience init(foo: Int)
+  convenience init(foo: Int) { self.init(somethingElse: ()) }
+  // FIXME: The '@objc' is relied upon, but the 'dynamic' probably shouldn't be!
+  // CHECK: {{^}} @objc convenience dynamic init(bar: Int)
+  convenience init(bar: Int) { self.init(somethingElse: ()) }
+
+  // CHECK: {{^}} convenience init(unrelated: Int)
+  convenience init(unrelated: Int) { self.init(somethingElse: ()) }
+} // CHECK: {{^[}]$}}
+
+// CHECK-LABEL: class DesignatedInitsInheritObjCAttrSub
+class DesignatedInitsInheritObjCAttrSub: InitsInheritObjCAttrBase {
+  // CHECK: {{^}} @objc init(foo: Int)
+  init(foo: Int) { super.init() }
+  // FIXME: The '@objc' is relied upon, but the 'dynamic' probably shouldn't be!
+  // CHECK: {{^}} @objc dynamic init(bar: Int)
+  init(bar: Int) { super.init() }
+  
+  // CHECK: {{^}} init(unrelated: Int)
+  init(unrelated: Int) { super.init() }
+} // CHECK: {{^[}]$}}