COFF: Add back an assertion that is superseded by r222124

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222735 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Object/COFFObjectFile.cpp

@@ -1060,7 +1060,7 @@ symbol_iterator COFFObjectFile::getRelocationSymbol(DataRefImpl Rel) const {
   else if (SymbolTable32)
     Ref.p = reinterpret_cast<uintptr_t>(SymbolTable32 + R->SymbolTableIndex);
   else
-    return symbol_end();
+    llvm_unreachable("no symbol table pointer!");
   return symbol_iterator(SymbolRef(Ref, this));
 }