[llvm] Add assertions for the smart pointers with the possibility to be null returned from ThinLTOCodeGenerator::linkCombinedIndex

[OikawaKirie]

Summary

Split from D91844.

The local variable Index in function ThinLTOCodeGenerator::run in file llvm/lib/LTO/ThinLTOCodeGenerator.cpp. When function ThinLTOCodeGenerator::linkCombinedIndex returns nullptr, the null smart pointer will be dereferenced below.

[steven_wu]

This one might be better just fix the FIXME in linkCombinedIndex since assert won't do anything in the release build.

Maybe instead of logAllUnhandledErrors, just report_fatal_error in linkCombindedIndex()? Then we should not get a nullptr from the function.

llvm/lib/LTO/ThinLTOCodeGenerator.cpp

for (auto &Mod : Modules) {
       }, count++);
     }
 
     return;
   }
 
   // Sequential linking phase
   auto Index = linkCombinedIndex();
+  assert(Index);
 
   // Save temps: index.
   if (!SaveTempsDir.empty()) {
     auto SaveTempPath = SaveTempsDir + "index.bc";
     std::error_code EC;
     raw_fd_ostream OS(SaveTempPath, EC, sys::fs::OF_None);
     if (EC)
       report_fatal_error(Twine("Failed to open ") + SaveTempPath +