[LIBCXX] std::ostream::sentry should be hidden

for testing by CI.
std::istream::sentry is left original to ensure test detects this defect.

Author: kikairoya

libcxx/include/__ostream/basic_ostream.h

@@ -186,8 +186,8 @@ class basic_ostream<_CharT, _Traits>::sentry {
   basic_ostream<_CharT, _Traits>& __os_;
 
 public:
-  explicit sentry(basic_ostream<_CharT, _Traits>& __os);
-  ~sentry();
+  explicit _LIBCPP_HIDE_FROM_ABI inline sentry(basic_ostream<_CharT, _Traits>& __os);
+  _LIBCPP_HIDE_FROM_ABI inline ~sentry();
   sentry(const sentry&)            = delete;
   sentry& operator=(const sentry&) = delete;