Stop relying on @_distributedActorIndependent anywhere.

This is an non-user-visible attribute that is semantically identical to
`nonisolated` except that it allows use in distributed actors. It is
only, and can only, be used for the two fields that distributed actors
have in both the local actor and in the remote proxy, "id" and
"actorTransport". So, special-case the "nonisolated" check for those
fields and stop using `@_distributedActorIndependent`.

Author: DougGregor

lib/SILGen/SILGenDistributed.cpp

@@ -817,7 +817,7 @@ void SILGenFunction::emitDistributedActorClassMemberDestruction(
     B.emitBlock(remoteMemberDestroyBB);
 
     for (VarDecl *vd : cd->getStoredProperties()) {
-      if (!vd->getAttrs().hasAttribute<DistributedActorIndependentAttr>())
+      if (getActorIsolation(vd) == ActorIsolation::DistributedActorInstance)
         continue;
 
       destroyClassMember(cleanupLoc, selfValue, vd);

lib/Sema/CodeSynthesisDistributedActor.cpp

@@ -172,9 +172,8 @@ AbstractFunctionDecl *TypeChecker::addImplicitDistributedActorRemoteFunction(
   remoteFuncDecl->getAttrs().add(
       new (C) DynamicAttr(/*implicit=*/true));
 
-  // @_distributedActorIndependent
-  remoteFuncDecl->getAttrs().add(
-      new (C) DistributedActorIndependentAttr(/*IsImplicit=*/true));
+  // nonisolated
+  remoteFuncDecl->getAttrs().add(new (C) NonisolatedAttr(/*IsImplicit=*/true));
 
   // users should never have to access this function directly;
   // it is only invoked from our distributed function thunk if the actor is remote.

lib/Sema/DerivedConformanceDistributedActor.cpp

@@ -119,9 +119,9 @@ static ValueDecl *deriveDistributedActor_id(DerivedConformance &derived) {
 
   propDecl->setIntroducer(VarDecl::Introducer::Let);
 
-  // mark as @_distributedActorIndependent, allowing access to it from everywhere
+  // mark as nonisolated, allowing access to it from everywhere
   propDecl->getAttrs().add(
-      new (C) DistributedActorIndependentAttr(/*IsImplicit=*/true));
+      new (C) NonisolatedAttr(/*IsImplicit=*/true));
 
   derived.addMembersToConformanceContext({ propDecl, pbDecl });
   return propDecl;
@@ -148,9 +148,9 @@ static ValueDecl *deriveDistributedActor_actorTransport(
 
   propDecl->setIntroducer(VarDecl::Introducer::Let);
 
-  // mark as @_distributedActorIndependent, allowing access to it from everywhere
+  // mark as nonisolated, allowing access to it from everywhere
   propDecl->getAttrs().add(
-      new (C) DistributedActorIndependentAttr(/*IsImplicit=*/true));
+      new (C) NonisolatedAttr(/*IsImplicit=*/true));
 
   derived.addMembersToConformanceContext({ propDecl, pbDecl });
   return propDecl;

lib/Sema/TypeCheckAttr.cpp

@@ -5473,7 +5473,13 @@ void AttributeChecker::visitNonisolatedAttr(NonisolatedAttr *attr) {
       // distributed actors. Attempts of nonisolated access would be
       // cross-actor, and that means they might be accessing on a remote actor,
       // in which case the stored property storage does not exist.
-      if (nominal && nominal->isDistributedActor()) {
+      //
+      // The synthesized "id" and "actorTransport" are the only exceptions,
+      // because the implementation mirrors them.
+      if (nominal && nominal->isDistributedActor() &&
+          !(var->isImplicit() &&
+            (var->getName() == Ctx.Id_id ||
+             var->getName() == Ctx.Id_actorTransport))) {
         diagnoseAndRemoveAttr(attr,
                               diag::nonisolated_distributed_actor_storage);
         return;