[BOLT] Support files with no symbols

[Amir]

Summary

LastSymbol handling in discoverFileObjects assumes a non-zero number of
symbols in an object file. It's not the case for broken_dynsym.test added in
D130073, and potentially other stripped binaries.

bolt/lib/Rewrite/RewriteInstance.cpp

auto CompareSymbols = [this](const SymbolRef &A, const SymbolRef &B) {
     if (BType == SymbolRef::ST_Debug && AType != SymbolRef::ST_Debug)
       return true;
 
     return false;
   };
 
   llvm::stable_sort(SortedFileSymbols, CompareSymbols);
 
-  auto LastSymbol = SortedFileSymbols.end() - 1;
+  auto LastSymbol = SortedFileSymbols.end();
+  if (!SortedFileSymbols.empty())
+    --LastSymbol;
 
   // For aarch64, the ABI defines mapping symbols so we identify data in the
   // code section (see IHI0056B). $d identifies data contents.
   // Compilers usually merge multiple data objects in a single $d-$x interval,
   // but we need every data object to be marked with $d. Because of that we
   // create a vector of MarkerSyms with all locations of data objects.
 
   struct MarkerSym {
auto addExtraDataMarkerPerSymbol =
         }
       };
 
   if (BC->isAArch64()) {
     addExtraDataMarkerPerSymbol(SortedFileSymbols, SortedMarkerSymbols);
     LastSymbol = std::stable_partition(
         SortedFileSymbols.begin(), SortedFileSymbols.end(),
         [this](const SymbolRef &Symbol) { return !BC->isMarker(Symbol); });
+    if (!SortedFileSymbols.empty())
       --LastSymbol;
   }
 
   BinaryFunction *PreviousFunction = nullptr;
   unsigned AnonymousId = 0;
 
-  const auto SortedSymbolsEnd = std::next(LastSymbol);
+  const auto SortedSymbolsEnd = LastSymbol == SortedFileSymbols.end()
+                                    ? LastSymbol
+                                    : std::next(LastSymbol);
   for (auto ISym = SortedFileSymbols.begin(); ISym != SortedSymbolsEnd;
        ++ISym) {
     const SymbolRef &Symbol = *ISym;
     // Keep undefined symbols for pretty printing?
     if (cantFail(Symbol.getFlags()) & SymbolRef::SF_Undefined)
       continue;
 
     const SymbolRef::Type SymbolType = cantFail(Symbol.getType());