Concurrency: add Windows support to SWIFT_TASK_PRINTF_DEBUG

`pthread_self` is not portable to all platforms.  Introduce a
`_swift_get_current_thread_id` to abstract over accessing the current
thread ID.  On Windows, the thread ID and thread handle are two separate
entities, unlike POSIX threads which treats them the same.

(cherry picked from commit 8c5d383)

Author: compnerd

stdlib/public/Concurrency/TaskPrivate.h

@@ -27,6 +27,16 @@
 
 #include "../runtime/StackAllocator.h"
 
+#if HAVE_PTHREAD_H
+#include <pthread.h>
+#endif
+#if defined(_WIN32)
+#define WIN32_LEAN_AND_MEAN
+#define VC_EXTRA_LEAN
+#define NOMINMAX
+#include <Windows.h>
+#endif
+
 namespace swift {
 
 // Uncomment to enable helpful debug spew to stderr