[Sema] Revert temporary downgrade of misplaced unownedExecutor diagnostic

rdar://78932296
(cherry picked from commit 6b07694)

Author: lorentey

include/swift/AST/DiagnosticsSema.def

@@ -4370,10 +4370,7 @@ ERROR(actor_protocol_illegal_inheritance,none,
       "non-actor type %0 cannot conform to the 'Actor' protocol",
       (DeclName))
 
-// FIXME: This diagnostic was temporarily downgraded from an error because
-// it spuriously triggers when building the Foundation module from its textual
-// swiftinterface. (rdar://78932296)
-WARNING(unowned_executor_outside_actor,none,
+ERROR(unowned_executor_outside_actor,none,
       "'unownedExecutor' can only be implemented within the main "
       "definition of an actor", ())
 ERROR(override_implicit_unowned_executor,none,

lib/Sema/TypeCheckProtocol.cpp

@@ -4584,10 +4584,7 @@ void ConformanceChecker::resolveValueWitnesses() {
           Adoptee->getClassOrBoundGenericClass() &&
           Adoptee->getClassOrBoundGenericClass()->isActor()) {
         witness->diagnose(diag::unowned_executor_outside_actor);
-        // FIXME: This diagnostic was temporarily downgraded from an error to a
-        // warning because it spuriously triggers when building the Foundation
-        // module from its textual swiftinterface. (rdar://78932296)
-        //return;
+        return;
       }
 
       // Objective-C checking for @objc requirements.