Fix for https://bugs.llvm.org/show_bug.cgi?id=36705

[AndreyChurbanov]

Summary

Debug build of the OpenMP RTL fails with assertion because of wrong th_reap_state flag during library shutdown.
The fix is to mark a thread SAFE_TO_REAP when it is put into the pool of free threads.

[tlwilmar]

LGTM

openmp/trunk/runtime/src/kmp_runtime.cpp

void __kmp_free_thread(kmp_info_t *this_th) {
   kmp_balign_t *balign = this_th->th.th_bar;
   for (b = 0; b < bs_last_barrier; ++b) {
     if (balign[b].bb.wait_flag == KMP_BARRIER_PARENT_FLAG)
       balign[b].bb.wait_flag = KMP_BARRIER_SWITCH_TO_OWN_FLAG;
     balign[b].bb.team = NULL;
     balign[b].bb.leaf_kids = 0;
   }
   this_th->th.th_task_state = 0;
+  this_th->th.th_reap_state = KMP_SAFE_TO_REAP;
 
   /* put thread back on the free pool */
   TCW_PTR(this_th->th.th_team, NULL);
   TCW_PTR(this_th->th.th_root, NULL);
   TCW_PTR(this_th->th.th_dispatch, NULL); /* NOT NEEDED */
 
   /* If the implicit task assigned to this thread can be used by other threads
    * -> multiple threads can share the data and try to free the task at