Fix most GCC attribute ignored warnings

llvm-svn: 267074
diff --git a/libcxx/include/thread b/libcxx/include/thread
index c3eb437..bf5b8e8 100644
--- a/libcxx/include/thread
+++ b/libcxx/include/thread
@@ -219,7 +219,7 @@
 
 }  // this_thread
 
-template<> struct _LIBCPP_TYPE_VIS_ONLY hash<__thread_id>;
+template<> struct hash<__thread_id>;
 
 class _LIBCPP_TYPE_VIS_ONLY __thread_id
 {