Define "Actors2" feature to cope with short-term backward compatibility

(cherry picked from commit 59b6563)

Author: DougGregor

include/swift/Basic/Features.def

@@ -39,6 +39,7 @@ LANGUAGE_FEATURE(AsyncAwait, 296, "async/await", true)
 LANGUAGE_FEATURE(EffectfulProp, 310, "Effectful properties", true)
 LANGUAGE_FEATURE(MarkerProtocol, 0, "@_marker protocol", true)
 LANGUAGE_FEATURE(Actors, 0, "actors", true)
+LANGUAGE_FEATURE(Actors2, 0, "actors #2 (TEMPORARY)", true)
 LANGUAGE_FEATURE(ConcurrentFunctions, 0, "@concurrent functions", true)
 LANGUAGE_FEATURE(RethrowsProtocol, 0, "@rethrows protocol", true)
 LANGUAGE_FEATURE(GlobalActors, 0, "Global actors", langOpts.EnableExperimentalConcurrency)

lib/AST/ASTPrinter.cpp

@@ -2607,6 +2607,10 @@ static bool usesFeatureConcurrentFunctions(Decl *decl) {
   return false;
 }
 
+static bool usesFeatureActors2(Decl *decl) {
+  return false;
+}
+
 static bool usesFeatureSendable(Decl *decl) {
   if (auto func = dyn_cast<AbstractFunctionDecl>(decl)) {
     if (func->isSendable())