Add a test that ensures tasks get invalidated

This detects a bug unique to the oldest back-deployment targets
for Swift concurrency, where Tasks were getting created with an
incorrect bit pattern that made them immortal.

Verifies that rdar://93087343 has been fixed.

Author: DougGregor

test/Concurrency/Runtime/task_destruction.swift

@@ -0,0 +1,34 @@
+// RUN: %target-run-simple-swift(-parse-as-library -Xfrontend -disable-availability-checking) | %FileCheck %s
+// REQUIRES: executable_test
+// REQUIRES: concurrency
+// REQUIRES: concurrency_runtime
+
+// UNSUPPORTED: back_deployment_runtime
+
+class C {}
+
+@_silgen_name("exit")
+func exit(_ code : UInt32) -> Never
+
+@main
+enum Main {
+  static func main() async {
+    weak var weakRef: C?
+    do {
+      let c = C()
+      let t = Task.detached { return c }
+      weakRef = c
+    }
+    Task.detached {
+      try await Task.sleep(nanoseconds: 10_000_000_000)
+      print("Fail!")
+      exit(1)
+    }
+    
+    while weakRef != nil {
+      await Task.yield()
+    }
+    // CHECK: Success
+    print("Success!")
+  }
+}