[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