WIP: Provide isolated actor for async init

Try to workaround swiftlang#71174, which crashes Swift with
an async initializer with isolated actor argument.

I have no idea what I am doing.

lib/SILGen/SILGenConcurrency.cpp

@@ -200,7 +200,15 @@ void SILGenFunction::emitConstructorPrologActorHop(
     SILLocation loc, std::optional<ActorIsolation> maybeIso) {
   loc = loc.asAutoGenerated();
   if (maybeIso) {
-    if (auto executor = emitExecutor(loc, *maybeIso, std::nullopt)) {
+    auto iso = *maybeIso;
+    std::optional<ManagedValue> maybeSelf;
+    if (iso == ActorIsolation::ActorInstance) {
+      auto actor = iso.getActorInstance();
+      Type actorType = actor->getTypeInContext();
+      RValue actorInstanceRV = emitRValueForDecl(loc, actor, actorType, AccessSemantics::Ordinary);
+      maybeSelf = std::move(actorInstanceRV).getScalarValue();
+    }
+    if (auto executor = emitExecutor(loc, iso, maybeSelf)) {
       ExpectedExecutor = *executor;
     }
   }