[Orc] Fix the SharedMemoryMapper dtor

[aganea]

Summary

As briefly discussed on rG1134d3a03fac, fix the unintended copy while iterating on Reservations and add a mutex guard, to be symmetric with other usages of Reservations.

[argentite]

Thanks! I forgot about this.

llvm/lib/ExecutionEngine/Orc/MemoryMapper.cpp

 #else
   OnReleased(make_error<StringError>(
       "SharedMemoryMapper is not supported on this platform yet",
       inconvertibleErrorCode()));
 #endif
 }
 
 SharedMemoryMapper::~SharedMemoryMapper() {
+  std::lock_guard<std::mutex> Lock(Mutex);
   for (const auto &R : Reservations) {
 
 #if defined(LLVM_ON_UNIX) && !defined(__ANDROID__)
 
     munmap(R.second.LocalAddr, R.second.Size);
 
 #elif defined(_WIN32)