Dimitry Andric:  many visibility fixes.  Howard:  Much appreciated.  Can you send me a patch to CREDITS.TXT?

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@163862 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/thread b/include/thread
index 94e7ab6..60d8885 100644
--- a/include/thread
+++ b/include/thread
@@ -173,13 +173,13 @@
     delete __p_old;
 }
 
-class thread;
-class __thread_id;
+class _LIBCPP_VISIBLE thread;
+class _LIBCPP_VISIBLE __thread_id;
 
 namespace this_thread
 {
 
-__thread_id get_id() _NOEXCEPT;
+_LIBCPP_INLINE_VISIBILITY __thread_id get_id() _NOEXCEPT;
 
 }  // this_thread