[gold/ThinLTO] Invoke llvm_shutdown when exiting after ThinLTO indexing

[tejohnson]

Summary

Instead of manually invoking PrintStatistics, simply invoke
llvm_shutdown which will take care of destroying managed statics, and as
a side effect will destroy the StatisticInfo ManagedStatic, invoking
PrintStatistics when needed.

[tejohnson]

BTW confirmed that the test that I added with the 2 lines being removed here fails when I don't have anything here, but passes again with the llvm_shutdown call.

[fhahn]

Nice thanks! LGTM

tools/gold/gold-plugin.cpp

static ld_plugin_status allSymbolsReadHook() {
 
   std::vector<std::pair<SmallString<128>, bool>> Files = runLTO();
 
   if (options::TheOutputType == options::OT_DISABLE ||
       options::TheOutputType == options::OT_BC_ONLY)
     return LDPS_OK;
 
   if (options::thinlto_index_only) {
-    if (llvm::AreStatisticsEnabled())
-      llvm::PrintStatistics();
+    llvm_shutdown();
     cleanup_hook();
     exit(0);
   }
 
   for (const auto &F : Files)
     if (!F.first.empty())
       recordFile(F.first.str(), F.second);