Include SwiftNativeNSObject in the back-deployed concurrency library.

SwiftNativeNSObject is part of the core runtime on OS versions that
also have the concurrency library, so we need to include a copy of
it with the back-deployed concurrency library.

Author: DougGregor

stdlib/public/BackDeployConcurrency/CMakeLists.txt

@@ -47,7 +47,8 @@ set(swift_concurrency_options
   DARWIN_INSTALL_NAME_DIR "@rpath")
 set(swift_concurrency_extra_sources
   "../BackDeployConcurrency/Exclusivity.cpp"
-  "../BackDeployConcurrency/Metadata.cpp")
+  "../BackDeployConcurrency/Metadata.cpp"
+  "../stubs/SwiftNativeNSObject.mm")
 set(swift_concurrency_async_fp_mode "never")
 
 add_subdirectory(../Concurrency stdlib/public/BackDeployConcurrency)

test/Concurrency/Backdeploy/mangling.swift

@@ -15,6 +15,7 @@
 // REQUIRES: CPU=x86_64
 // REQUIRES: OS=macosx
 // REQUIRES: executable_test
+// REQUIRES: concurrency_runtime
 actor MyActor { }
 
 protocol MyProtocol {

test/Concurrency/Backdeploy/objc_actor.swift

@@ -0,0 +1,23 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-build-swift -target x86_64-apple-macosx10.15 %s -o %t/test_mangling -Xfrontend -disable-availability-checking -parse-as-library
+// RUN: %target-run %t/test_mangling
+
+// REQUIRES: CPU=x86_64
+// REQUIRES: OS=macosx
+// REQUIRES: executable_test
+// REQUIRES: concurrency_runtime
+
+import Foundation
+
+@objc actor MyActor {
+  func f() -> String { "hello" }
+}
+
+@main
+enum Main {
+  static func main() async {
+    let ma = MyActor()
+    let greeting = await ma.f()
+    assert(greeting == "hello")
+  }
+}