[Distributed] Avoid cascading diagnostics when actorSystem is declared

explicitly.

Author: hborla

lib/Sema/TypeCheckDistributed.cpp

@@ -681,6 +681,7 @@ void swift::checkDistributedActorProperties(const NominalTypeDecl *decl) {
       if (id == C.Id_actorSystem || id == C.Id_id) {
         prop->diagnose(diag::distributed_actor_user_defined_special_property,
                       id);
+        prop->setInvalid();
       }
     }
   }

test/decl/protocol/special/DistributedActor.swift

@@ -38,14 +38,12 @@ distributed actor D2 {
   // expected-error@-1{{actor 'D2' has no initializers}}
   let actorSystem: String
   // expected-error@-1{{property 'actorSystem' cannot be defined explicitly, as it conflicts with distributed actor synthesized stored property}}
-  // expected-error@-2{{invalid redeclaration of synthesized implementation for protocol requirement 'actorSystem'}}
-  // expected-note@-3{{stored property 'actorSystem' without initial value prevents synthesized initializers}}
+  // expected-note@-2{{stored property 'actorSystem' without initial value prevents synthesized initializers}}
 }
 
 distributed actor D3 {
   var id: Int { 0 }
   // expected-error@-1{{property 'id' cannot be defined explicitly, as it conflicts with distributed actor synthesized stored property}}
-  // expected-error@-2{{invalid redeclaration of synthesized implementation for protocol requirement 'id'}}
 }
 
 struct OtherActorIdentity: Sendable, Hashable, Codable {}
@@ -55,12 +53,10 @@ distributed actor D4 {
 
   let actorSystem: String
   // expected-error@-1{{property 'actorSystem' cannot be defined explicitly, as it conflicts with distributed actor synthesized stored property}}
-  // expected-error@-2{{invalid redeclaration of synthesized implementation for protocol requirement 'actorSystem'}}
-  // expected-note@-3{{stored property 'actorSystem' without initial value prevents synthesized initializers}}
+  // expected-note@-2{{stored property 'actorSystem' without initial value prevents synthesized initializers}}
   let id: OtherActorIdentity
   // expected-error@-1{{property 'id' cannot be defined explicitly, as it conflicts with distributed actor synthesized stored property}}
-  // expected-error@-2{{invalid redeclaration of synthesized implementation for protocol requirement 'id'}}
-  // expected-note@-3{{stored property 'id' without initial value prevents synthesized initializers}}
+  // expected-note@-2{{stored property 'id' without initial value prevents synthesized initializers}}
 }
 
 protocol P1: DistributedActor {