[MCJIT] Re-enable GDB registration (temporarily disabled in r222811), but check

that we actually have an object to register first.

For MachO objects, RuntimeDyld::LoadedObjectInfo::getObjectForDebug returns an
empty OwningBinary<ObjectFile> which was causing crashes in the GDB registration
code.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222812 91177308-0d34-0410-b5e6-96231b3b80d8

lib/ExecutionEngine/MCJIT/MCJIT.cpp

@@ -75,6 +75,7 @@ MCJIT::MCJIT(std::unique_ptr<Module> M, std::unique_ptr<TargetMachine> tm,
 
   OwnedModules.addModule(std::move(First));
   setDataLayout(TM->getSubtargetImpl()->getDataLayout());
+  RegisterJITEventListener(JITEventListener::createGDBRegistrationListener());
 }
 
 MCJIT::~MCJIT() {