[Concurrency] Implicit global actor attributes imply Sendable.

Sendable derivation was only checking for explicit global actor
attributes. It should also account for inferred global actor
attributes, e.g. from protocol conformances.

lib/Sema/TypeCheckConcurrency.cpp

@@ -6055,7 +6055,7 @@ ProtocolConformance *swift::deriveImplicitSendableConformance(
   }
 
   // A non-protocol type with a global actor is implicitly Sendable.
-  if (nominal->getGlobalActorAttr()) {
+  if (getActorIsolation(nominal).isGlobalActor()) {
     // Form the implicit conformance to Sendable.
     return formConformance(nullptr);
   }

test/Concurrency/sendable_checking.swift

@@ -407,3 +407,23 @@ struct DowngradeForPreconcurrency {
     }
   }
 }
+
+@available(SwiftStdlib 5.1, *)
+@MainActor protocol InferMainActor {}
+
+@available(SwiftStdlib 5.1, *)
+struct ImplicitSendableViaMain: InferMainActor {}
+
+@available(SwiftStdlib 5.1, *)
+extension ImplicitSendableViaMain {
+  nonisolated func capture() {
+    Task { @MainActor in
+      _ = self
+    }
+  }
+}
+
+@available(SwiftStdlib 5.1, *)
+struct TestImplicitSendable: Sendable {
+  var x: ImplicitSendableViaMain
+}