[typechecker] allow @objc functions to return dynamic self

Author: Suyash Srijan

lib/Sema/TypeCheckDeclObjC.cpp

@@ -523,6 +523,7 @@ bool swift::isRepresentableInObjC(
     if (!ResultType->hasError() &&
         !ResultType->isVoid() &&
         !ResultType->isUninhabited() &&
+        !ResultType->hasDynamicSelfType() &&
         !ResultType->isRepresentableIn(ForeignLanguage::ObjectiveC,
                                        const_cast<FuncDecl *>(FD))) {
       if (Diagnose) {

test/ClangImporter/objc_parse.swift

@@ -273,8 +273,7 @@ func classAnyObject(_ obj: NSObject) {
 class Wobbler : NSWobbling {
   @objc func wobble() { }
 
-  func returnMyself() -> Self { return self } // expected-error{{non-'@objc' method 'returnMyself()' does not satisfy requirement of '@objc' protocol 'NSWobbling'}}{{none}}
-  // expected-error@-1{{method cannot be an implementation of an @objc requirement because its result type cannot be represented in Objective-C}}
+  func returnMyself() -> Self { return self }
 }
 
 extension Wobbler : NSMaybeInitWobble { // expected-error{{type 'Wobbler' does not conform to protocol 'NSMaybeInitWobble'}}

test/attr/attr_objc.swift

@@ -323,7 +323,6 @@ class ConcreteContext3 {
   func dynamicSelf1() -> Self { return self }
 
   @objc func dynamicSelf1_() -> Self { return self }
-  // expected-error@-1{{method cannot be marked @objc because its result type cannot be represented in Objective-C}}
 
   @objc func genericParams<T: NSObject>() -> [T] { return [] }
   // expected-error@-1{{method cannot be marked @objc because it has generic parameters}}