[llvm-objcopy][MachO] Recompute and update offset/size fields in the writer
Recompute and update offset/size fields so that we can implement llvm-objcopy options like --only-section.
This patch is the first step and focuses on supporting load commands that covered by existing tests: executable files and
dynamic libraries are not supported.
Reviewers: alexshap, rupprecht, jhenderson
Reviewed By: alexshap, rupprecht
Subscribers: compnerd, jakehehrlich, llvm-commits
Differential Revision: https://reviews.llvm.org/D62652