Effort to reduce the number of exported symbols

llvm-svn: 122057
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: c950e77d1d0431b9fa9144e9ea7eaa858d8e032e
diff --git a/include/thread b/include/thread
index abb56f4..8b3edb7 100644
--- a/include/thread
+++ b/include/thread
@@ -297,7 +297,7 @@
 
 class __assoc_sub_state;
 
-class __thread_struct_imp;
+class _LIBCPP_HIDDEN __thread_struct_imp;
 
 class __thread_struct
 {