Add support for chained fixup load commands to MachOObjectFile

This is part of a series of patches to upstream support for Mach-O chained fixups.

This patch adds support for parsing the chained fixup load command and
parsing the chained fixups header. It also puts into place the
abstract interface that will be used to iterate over the fixups.

Differential Revision: https://reviews.llvm.org/D113630