[LTO] Return proper error object rather than null LTOModule

This caused a crash in LTOModule::createInLocalContext.

rdar://37926841

llvm-svn: 327359

llvm/lib/LTO/LTOModule.cpp

@@ -208,7 +208,7 @@ LTOModule::makeLTOModule(MemoryBufferRef Buffer, const TargetOptions &options,
   std::string errMsg;
   const Target *march = TargetRegistry::lookupTarget(TripleStr, errMsg);
   if (!march)
-    return std::unique_ptr<LTOModule>(nullptr);
+    return make_error_code(object::object_error::arch_not_found);
 
   // construct LTOModule, hand over ownership of module and target
   SubtargetFeatures Features;