[Concurrency/Distributed] nonisolated-nonsending by default breaks distributed thunks

the new NonisolatedNonsendingByDefault upcoming feature breaks remote
calls in distributed actors, because the expected isolation doesn't
match and the runtime swift_distributed_execute_target_resume will
crash.

This is a short term fix to unblock adopters, however preferably we
should mark the thunks as nonisolated(nonsending), though that seems to
be more involved.

resolves rdar://159247975

lib/Sema/CodeSynthesisDistributedActor.cpp

@@ -737,6 +737,7 @@ static FuncDecl *createSameSignatureDistributedThunkDecl(DeclContext *DC,
 
   thunk->setSynthesized(true);
   thunk->setDistributedThunk(true);
+  // TODO(distributed): These would benefit from becoming nonisolated(nonsending)
   thunk->getAttrs().add(NonisolatedAttr::createImplicit(C));
 
   return thunk;