[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.

(cherry picked from commit e044e3d)

Author: hborla

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
+}