[llvm-nm][MachO] Add support for `MH_FILESET`

Support printing of symbols for MachO of `MH_FILESET` type.
This is achieved by extending `dumpSymbolNamesFromObject`
to encompass fileset handling, and including an offset in
`MachOObjectFile` class to locate embedded MachO headers.

Differential Revision: https://reviews.llvm.org/D159294