Run objc-arc-contract when arc module flag is set

[steven_wu]

Summary

objc-arc-contract is running unconditionally now in LTO. Since we
introduced the module flag for ARC in clang, we only need to run
that when the module flag is set.

This is relaying on the change in http://reviews.llvm.org/D7799
Not sure if I should add the pass to llc as well.

lib/LTO/LTOCodeGenerator.cpp

bool LTOCodeGenerator::compileOptimized(raw_ostream &out, std::string &errMsg) {
 
   legacy::PassManager codeGenPasses;
 
   codeGenPasses.add(new DataLayoutPass());
 
   formatted_raw_ostream Out(out);
 
   // If the bitcode files contain ARC code and were compiled with optimization,
-  // the ObjCARCContractPass must be run, so do it unconditionally here.
+  // the ObjCARCContractPass must be run, check the module flag to see if there
+  // is any file requires ARC
+  if (mergeModule->getModuleFlag("Objective-C ARC"))
     codeGenPasses.add(createObjCARCContractPass());
 
   if (TargetMach->addPassesToEmitFile(codeGenPasses, Out,
                                       TargetMachine::CGFT_ObjectFile)) {
     errMsg = "target file type not supported";
     return false;
   }
 
   // Run the code generator, and write assembly file