[dwarf][NFC] Move expandBundle() to MachO.h

The function `expandBundle()` is defined both in `llvm-dwarfdump.cpp` and
`llvm-gsymutil.cpp` in the exact same way. Reduce code duplication by
moving this function to the `MachOObjectFile` class.

Reviewed By: jhenderson, clayborg

Differential Revision: https://reviews.llvm.org/D115418