Use the distributed thunk for non-distributed protocol requirements (#59513)

When a distributed-actor-isolated witness is provided for a
non-distributed-actor-isolated requirement, always hop to the
distributed thunk.

While here, mark distributed thunks as being `final`, to ensure that
we always statically call them vs. trying to retrieve them from the
vtable (since they aren't in the vtable).

lib/Sema/CodeSynthesisDistributedActor.cpp

@@ -699,6 +699,10 @@ static FuncDecl *createDistributedThunkFunction(FuncDecl *func) {
       func->getResultInterfaceType(), DC);
   thunk->setSynthesized(true);
   thunk->getAttrs().add(new (C) NonisolatedAttr(/*isImplicit=*/true));
+
+  if (isa<ClassDecl>(DC))
+    thunk->getAttrs().add(new (C) FinalAttr(/*isImplicit=*/true));
+
   thunk->setGenericSignature(baseSignature);
   thunk->copyFormalAccessFrom(func, /*sourceIsParentContext=*/false);
   thunk->setBodySynthesizer(deriveBodyDistributed_thunk, func);