[PrintAsObjC] Use 'unsafe_unretained' to print 'unowned', not 'assign'

The ObjC generator previously preserved a subtle difference between
'unowned' and 'unowned(unsafe)' / Unmanaged by printing the former as
'assign' and the latter as 'unsafe_unretained'. Upstream Clang,
however, has gotten a new warning to discourage the use of 'assign'
with reference-countable types at all. Since it was always a subtle
distinction, just go with the new convention and print
'unsafe_unretained' for 'unowned' properties as well.

rdar://problem/44290715

Author: jrose-apple

lib/PrintAsObjC/PrintAsObjC.cpp

@@ -1065,9 +1065,6 @@ class ObjCPrinter : private DeclVisitor<ObjCPrinter>,
       os << ", readonly";
 
     // Print the ownership semantics, if relevant.
-    // We treat "unowned" as "assign" (even though it's more like
-    // "safe_unretained") because we want people to think twice about
-    // allowing that object to disappear.
     Type ty = VD->getInterfaceType();
     if (auto weakTy = ty->getAs<WeakStorageType>()) {
       auto innerTy = weakTy->getReferentType()->getOptionalObjectType();
@@ -1077,9 +1074,11 @@ class ObjCPrinter : private DeclVisitor<ObjCPrinter>,
           (innerTy->isObjCExistentialType() && !isCFTypeRef(innerTy))) {
         os << ", weak";
       }
-    } else if (ty->is<UnownedStorageType>()) {
-      os << ", assign";
-    } else if (ty->is<UnmanagedStorageType>()) {
+    } else if (ty->is<UnownedStorageType>()|| ty->is<UnmanagedStorageType>()) {
+      // We treat "unowned" as "unsafe_unretained" (even though it's more like
+      // "safe_unretained") because we want people to think twice about
+      // allowing that object to disappear. "unowned(unsafe)" (and Unmanaged,
+      // handled below) really are "unsafe_unretained".
       os << ", unsafe_unretained";
     } else {
       Type copyTy = ty;

test/PrintAsObjC/classes.swift

@@ -503,7 +503,7 @@ public class NonObjCClass { }
 // CHECK-NEXT: SWIFT_CLASS_PROPERTY(@property (nonatomic, class, readonly, strong) Properties * _Nonnull sharedRO;)
 // CHECK-NEXT: + (Properties * _Nonnull)sharedRO SWIFT_WARN_UNUSED_RESULT;
 // CHECK-NEXT: @property (nonatomic, weak) Properties * _Nullable weakOther;
-// CHECK-NEXT: @property (nonatomic, assign) Properties * _Nonnull unownedOther;
+// CHECK-NEXT: @property (nonatomic, unsafe_unretained) Properties * _Nonnull unownedOther;
 // CHECK-NEXT: @property (nonatomic, unsafe_unretained) Properties * _Nonnull unmanagedOther;
 // CHECK-NEXT: @property (nonatomic, unsafe_unretained) Properties * _Nullable unmanagedByDecl;
 // CHECK-NEXT: @property (nonatomic, weak) id <MyProtocol> _Nullable weakProto;

test/PrintAsObjC/protocols.swift

@@ -191,7 +191,7 @@ extension NSString : A, ZZZ {}
 @objc class Subclass : RootClass1, ZZZ {}
 
 // CHECK-LABEL: @protocol UnownedProperty
-// CHECK-NEXT: @property (nonatomic, assign) id _Nonnull unownedProp;
+// CHECK-NEXT: @property (nonatomic, unsafe_unretained) id _Nonnull unownedProp;
 @objc protocol UnownedProperty {
   unowned var unownedProp: AnyObject { get set }
 }