[llvm-objcopy][MachO] Support indirect symbol table

Summary:
Parse the indirect symbol table and update the indexes of
symbol entries in the table in the writer in case they have
been changed.

Reviewers: alexshap, rupprecht, jhenderson

Reviewed By: alexshap, rupprecht

Subscribers: jakehehrlich, abrachet, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66280