Make folly::getCurrentThreadId() return a thread ID on OSX

Summary: It was previously returning a pointer masquerading as a `uint64_t`.

Reviewed By: yfeldblum

Differential Revision: D4888325

fbshipit-source-id: 6cf6ed09f0e7a39a8a2f93d3fa14b06913c27805

folly/ThreadId.h

@@ -24,7 +24,9 @@
 namespace folly {
 
 inline uint64_t getCurrentThreadID() {
-#ifdef _WIN32
+#if __APPLE__
+  return uint64_t(pthread_mach_thread_np(pthread_self()));
+#elif _WIN32
   return uint64_t(GetCurrentThreadId());
 #else
   return uint64_t(pthread_self());