[mach-o] Pass vectors by reference and name empty vector.
Add BumpPtrAllocator to lld::File.  Switch SimpleDefinedAtom to allocate
its SimpleRefernces using the BumpPtrAllocator.
[mach-o] Switch MachOFile and MachODylibFile to use BumpPtr in lld::File
[mach-o] fix leak in atoms -> normalized