[AST/Sema] Hide using declaration behind DefaultIsolationPerFile experimental feature

lib/AST/FeatureSet.cpp

@@ -643,6 +643,10 @@ static bool usesFeatureAlwaysInheritActorContext(Decl *decl) {
   return false;
 }
 
+static bool usesFeatureDefaultIsolationPerFile(Decl *D) {
+  return isa<UsingDecl>(D);
+}
+
 UNINTERESTING_FEATURE(BuiltinSelect)
 
 // ----------------------------------------------------------------------------