Fix compilation error with older libstdc++

[vadimcn]

Summary

Until GCC 6, std::unordered_map could not emplace non-copyable values, which results in this error: "error: call to implicitly-deleted copy constructor of 'lldb_private::ThreadPlanStack'" (due to ThreadPlanStack::m_stack_mutex not being copyable). Using std::piecewise_construct works around this problem.

[vadimcn]

Re-formatted

lldb/include/lldb/Target/ThreadPlanStack.h

 
   void AddThread(Thread &thread) {
     lldb::tid_t tid = thread.GetID();
-    m_plans_list.emplace(tid, thread);
+    m_plans_list.emplace(std::piecewise_construct, std::forward_as_tuple(tid),
+                         std::forward_as_tuple(thread));
   }
 
   bool RemoveTID(lldb::tid_t tid) {