[Concurrency] Fix ObjC weak references to actors.

Change actor destruction to call swift_deallocClassInstance instead of swift_deallocObject. When ObjC interop is enabled, swift_deallocClassInstance will check the "pure swift deallocation" bit and call into the ObjC runtime to destruct the instance when needed. This is what clears weak references and associated objects.

rdar://91270492

Author: mikeash

stdlib/public/Concurrency/Actor.cpp

@@ -1208,8 +1208,8 @@ void DefaultActorImpl::deallocateUnconditional() {
   if (JobStorageHeapObject.metadata != nullptr)
     JobStorage.~ProcessInlineJob();
   auto metadata = cast<ClassMetadata>(this->metadata);
-  swift_deallocObject(this, metadata->getInstanceSize(),
-                      metadata->getInstanceAlignMask());
+  swift_deallocClassInstance(this, metadata->getInstanceSize(),
+                             metadata->getInstanceAlignMask());
 }
 
 void DefaultActorImpl::scheduleActorProcessJob(JobPriority priority, bool useInlineJob) {

test/Interpreter/SDK/weak_objc_actor_interop.swift

@@ -0,0 +1,35 @@
+// RUN: %empty-directory(%t)
+//
+// RUN: cp %s %t/main.swift
+// RUN: %target-clang -fno-objc-arc %S/Inputs/ObjCWeak/ObjCWeak.m -c -o %t/ObjCWeak.o
+// RUN: %target-build-swift %t/main.swift -I %S/Inputs/ObjCWeak/ -Xlinker %t/ObjCWeak.o -o %t/weak_objc_actor_interop -Xfrontend -disable-access-control
+// RUN: %target-codesign %t/weak_objc_actor_interop
+// RUN: %target-run %t/weak_objc_actor_interop 2>&1 | %FileCheck %s
+// REQUIRES: executable_test
+// REQUIRES: objc_interop
+// REQUIRES: concurrency
+// UNSUPPORTED: use_os_stdlib
+// UNSUPPORTED: back_deployment_runtime
+
+import Foundation
+import ObjCWeak
+
+// Test that instances of Swift actors can be referenced using ObjC
+// weak references.
+
+@available(SwiftStdlib 5.1, *)
+actor A {
+  @objc nonisolated var description: String {
+    return "Swift Actor"
+  }
+}
+
+// This availability check will always be true, so we don't need to provide
+// FileCheck with fake output in an else branch.
+if #available(SwiftStdlib 5.1, *) {
+  tryWeakReferencing { A() }
+  // CHECK:       before giving up strong reference:
+  // CHECK-NEXT:  Swift Actor
+  // CHECK-NEXT:  after giving up strong reference:
+  // CHECK-NEXT:  Gone
+}