[Orc] Reorder operations in ExecutorSharedMemoryMapperService shutdown

[argentite]

This should fix https://reviews.llvm.org/D128544#3710127 but I think we may not even need the lock at all since this is called only at shutdown.

[lhames]

LGTM. Thanks Argentite!

llvm/lib/ExecutionEngine/Orc/TargetProcess/ExecutorSharedMemoryMapperService.cpp

#else
   return make_error<StringError>(
       "SharedMemoryMapper is not supported on this platform yet",
       inconvertibleErrorCode());
 #endif
 }
 
 Error ExecutorSharedMemoryMapperService::shutdown() {
   std::vector<ExecutorAddr> ReservationAddrs;
-  if (!Reservations.empty()) {
-    std::lock_guard<std::mutex> Lock(Mutex);
-    {
-      ReservationAddrs.reserve(Reservations.size());
-      for (const auto &R : Reservations) {
-        ReservationAddrs.push_back(ExecutorAddr::fromPtr(R.getFirst()));
-      }
-    }
-  }
-  return release(ReservationAddrs);
+    std::lock_guard<std::mutex> Lock(Mutex);
+
+    if (Reservations.empty())
+      return Error::success();
+
+
+    Reservations.clear();
 
-  return Error::success();
+  return release(ReservationAddrs);
 }
 
 void ExecutorSharedMemoryMapperService::addBootstrapSymbols(
     StringMap<ExecutorAddr> &M) {
   M[rt::ExecutorSharedMemoryMapperServiceInstanceName] =
       ExecutorAddr::fromPtr(this);
   M[rt::ExecutorSharedMemoryMapperServiceReserveWrapperName] =
       ExecutorAddr::fromPtr(&reserveWrapper);