[Concurrency] Don't let "unsafe" inference block other inference.

The presence of an unsafe global-actor attribute on something we can
infer from (e.g., a witnessed protocol requirement or overridden
declaration) was preventing propagation of actor-isolation attributes
from outer scopes. Only take "positive" results from such places.

Author: DougGregor

lib/Sema/TypeCheckConcurrency.cpp

@@ -2669,7 +2669,8 @@ ActorIsolation ActorIsolationRequest::evaluate(
     // If the declaration witnesses a protocol requirement that is isolated,
     // use that.
     if (auto witnessedIsolation = getIsolationFromWitnessedRequirements(value)) {
-      return inferredIsolation(*witnessedIsolation);
+      if (auto inferred = inferredIsolation(*witnessedIsolation))
+        return inferred;
     }
 
     // If the declaration is a class with a superclass that has specified
@@ -2688,7 +2689,8 @@ ActorIsolation ActorIsolationRequest::evaluate(
             superclassIsolation = superclassIsolation.subst(subs);
           }
 
-          return inferredIsolation(superclassIsolation);
+          if (auto inferred = inferredIsolation(superclassIsolation))
+            return inferred;
         }
       }
     }

test/Concurrency/global_actor_inference.swift

@@ -361,6 +361,13 @@ struct StructUGA2: UGA {
   nonisolated func req() { }
 }
 
+@SomeGlobalActor
+struct StructUGA3: UGA {
+  func req() {
+    sibling()
+  }
+}
+
 @GenericGlobalActor<String>
 func testUGA<T: UGA>(_ value: T) {
   value.req() // expected-error{{instance method 'req()' isolated to global actor 'SomeGlobalActor' can not be referenced from different global actor 'GenericGlobalActor<String>'}}