[BUG] llvm-libc++/thread could exhaust the process pthread_key_t

[WheelsMaker]

When I use the dynamic link file with dlopen and dlclose, the process' pthread_key_t will be exhausted and the process will crash when call std::thread.
Because the destructor , in the file thread, is empty,__thread_specific_ptr<_Tp>::~__thread_specific_ptr.

In fact, the destructor is not necessary left to be empty, because there only one static __thread_specific_ptr<_Tp> object in the dynamic link file, which is defined in the function __thread_local_data(). When calling dlclose()，the static object will be released and the pthread_key_t should be released too.

[DanAlbert]

#789

The best solution is to not call dlclose. It does more harm than good.