[Actor isolation] Fix a crash involving global actors, ObjC, and conformances

lib/Sema/TypeCheckProtocol.cpp

@@ -2749,8 +2749,12 @@ bool ConformanceChecker::checkActorIsolation(
   //
   // However, we allow this case when the requirement was imported, because
   // it might not have been annotated.
-  if (witnessGlobalActor && !requirementGlobalActor &&
-      !requirement->hasClangNode()) {
+  if (witnessGlobalActor && !requirementGlobalActor) {
+    // If the requirement was imported from Objective-C, it may not have been
+    // annotated appropriately. Allow the mismatch.
+    if (requirement->hasClangNode())
+      return false;
+
     witness->diagnose(
         diag::global_actor_isolated_witness, witness->getDescriptiveKind(),
         witness->getName(), witnessGlobalActor, Proto->getName());

test/ClangImporter/objc_async_conformance.swift

@@ -64,4 +64,13 @@ class SelectorOK2 : NSObject, RequiredObserverOnlyCompletion {
 class Rock : NSObject, Rollable {
   func roll(completionHandler: @escaping () -> Void) { completionHandler() }
   func roll() { roll(completionHandler: {}) }
-}
+}
+
+// Crash involving actor isolation checking.
+class C5 {
+  @MainActor @objc var allOperations: [String] = []
+}
+
+class C6: C5, ServiceProvider {
+  @MainActor func allOperations() async -> [String] { [] }
+}