[5.5][Concurrency] Expose Job's metadata as a debug variable

(cherry picked from commit e4a1d2b)

Author: mikeash

stdlib/public/Concurrency/Debug.h

@@ -21,6 +21,13 @@
 
 namespace swift {
 
+// Dispatch knows about these symbol names. Don't change them without consulting
+// dispatch.
+
+/// The metadata pointer used for job objects.
+SWIFT_RUNTIME_STDLIB_SPI
+const void *const _swift_concurrency_debug_jobMetadata;
+
 /// The metadata pointer used for async task objects.
 SWIFT_RUNTIME_STDLIB_SPI
 const void *const _swift_concurrency_debug_asyncTaskMetadata;

stdlib/public/Concurrency/Task.cpp

@@ -247,6 +247,8 @@ static FullMetadata<DispatchClassMetadata> taskHeapMetadata = {
   }
 };
 
+const void *const swift::_swift_concurrency_debug_jobMetadata =
+    static_cast<Metadata *>(&jobHeapMetadata);
 const void *const swift::_swift_concurrency_debug_asyncTaskMetadata =
     static_cast<Metadata *>(&taskHeapMetadata);