[Executors] isStdlibDefaultImplDecl must guard from null values, coming from Backdeploy library

resolves rdar://114029779

Author: ktoso

lib/Sema/TypeCheckConcurrency.cpp

@@ -1360,6 +1360,9 @@ void swift::tryDiagnoseExecutorConformance(ASTContext &C,
 
   auto concurrencyModule = C.getLoadedModule(C.Id_Concurrency);
   auto isStdlibDefaultImplDecl = [executorDecl, concurrencyModule](ValueDecl *witness) -> bool {
+    if (!witness)
+      return false;
+
     if (auto declContext = witness->getDeclContext()) {
       if (auto *extension = dyn_cast<ExtensionDecl>(declContext)) {
         auto extensionModule = extension->getParentModule();

test/Inputs/clang-importer-sdk/swift-modules-concurrency-without-job/_Concurrency.swift

@@ -1,10 +1,15 @@
 
 // This simulates a pre-Swift5.9 concurrency library with `Job` and `ExecutorJob` not being defined at all.
 // This is used to verify missing type handling in the SDK when a latest compiler is used.
+//
+// This also looks exactly like the Back deployment library which also does not
+// know about Job, and therefore is missing the Job based (and ExecutorJob based),
+// protocol requirements. This helps confirm we don't crash assuming that those
+// protocol requirements would always be there.
 
 public struct UnownedJob {}
 
-public protocol SerialExecutor {
-  // pretend to be a broken, empty serial executor
+public protocol Executor {
   func enqueue(_ job: UnownedJob)
-}
+}
+public protocol SerialExecutor: Executor { }