[Concurrency] Make actor-isolated protocol witnesses an error.

With actor isolation checking for protocol witnesses moved out of the
witness-matching phase, move the corresponding diagnostics from notes
(that would have been on the "type does not conform" error) to
freestanding errors.

Author: DougGregor

include/swift/AST/DiagnosticsSema.def

@@ -4172,10 +4172,10 @@ NOTE(actor_mutable_state,none,
 WARNING(shared_mutable_state_access,none,
         "reference to %0 %1 is not concurrency-safe because it involves "
         "shared mutable state", (DescriptiveDeclKind, DeclName))
-NOTE(actor_isolated_witness,none,
+ERROR(actor_isolated_witness,none,
      "actor-isolated %0 %1 cannot be used to satisfy a protocol requirement",
      (DescriptiveDeclKind, DeclName))
-NOTE(actor_isolated_witness_could_be_async_handler,none,
+ERROR(actor_isolated_witness_could_be_async_handler,none,
      "actor-isolated %0 %1 cannot be used to satisfy a protocol requirement; "
      "did you mean to make it an asychronous handler?",
      (DescriptiveDeclKind, DeclName))

test/decl/class/actor/conformance.swift

@@ -12,37 +12,32 @@ extension MyActor: AsyncProtocol {
   func asyncMethod() async -> Int { return 0 }
 }
 
-// FIXME: "Do you want to add a stub?" diagnostics should be suppressed here.
 protocol SyncProtocol {
   var propertyA: Int { get }
-  // expected-note@-1{{do you want to add a stub}}
   var propertyB: Int { get set }
-  // expected-note@-1{{do you want to add a stub}}
 
   func syncMethodA()
-  // expected-note@-1{{do you want to add a stub}}
 
   func syncMethodB()
 
   func syncMethodC() -> Int
 
   subscript (index: Int) -> String { get }
-  // expected-note@-1{{do you want to add a stub}}
 
   static func staticMethod()
   static var staticProperty: Int { get }
 }
 
 
-actor class OtherActor: SyncProtocol { // expected-error{{type 'OtherActor' does not conform to protocol 'SyncProtocol'}}
+actor class OtherActor: SyncProtocol {
   var propertyB: Int = 17
-  // expected-note@-1{{actor-isolated property 'propertyB' cannot be used to satisfy a protocol requirement}}
+  // expected-error@-1{{actor-isolated property 'propertyB' cannot be used to satisfy a protocol requirement}}
 
   var propertyA: Int { 17 }
-  // expected-note@-1{{actor-isolated property 'propertyA' cannot be used to satisfy a protocol requirement}}
+  // expected-error@-1{{actor-isolated property 'propertyA' cannot be used to satisfy a protocol requirement}}
 
   func syncMethodA() { }
-  // expected-note@-1{{actor-isolated instance method 'syncMethodA()' cannot be used to satisfy a protocol requirement; did you mean to make it an asychronous handler?}}{{3-3=@asyncHandler }}
+  // expected-error@-1{{actor-isolated instance method 'syncMethodA()' cannot be used to satisfy a protocol requirement; did you mean to make it an asychronous handler?}}{{3-3=@asyncHandler }}
 
   // Async handlers are okay.
   @asyncHandler
@@ -53,7 +48,7 @@ actor class OtherActor: SyncProtocol { // expected-error{{type 'OtherActor' does
   @actorIndependent func syncMethodC() -> Int { 5 }
 
   subscript (index: Int) -> String { "\(index)" }
-  // expected-note@-1{{actor-isolated subscript 'subscript(_:)' cannot be used to satisfy a protocol requirement}}
+  // expected-error@-1{{actor-isolated subscript 'subscript(_:)' cannot be used to satisfy a protocol requirement}}
 
   // Static methods and properties are okay.
   static func staticMethod() { }

test/decl/protocol/special/Actor.swift

@@ -42,7 +42,7 @@ class C1: Actor {
 
 // Method that is not usable as a witness.
 actor class BA1 {
-  func enqueue(partialTask: PartialAsyncTask) { } // expected-note{{actor-isolated instance method 'enqueue(partialTask:)' cannot be used to satisfy a protocol requirement; did you mean to make it an asychronous handler?}}
+  func enqueue(partialTask: PartialAsyncTask) { } // expected-error{{actor-isolated instance method 'enqueue(partialTask:)' cannot be used to satisfy a protocol requirement; did you mean to make it an asychronous handler?}}
 }
 
 // Method that isn't part of the main class definition cannot be used to