[NFC][llvm-libtool-darwin] Encapsulate the process of adding a new member in a class

Here we are refactoring the code to encapsulate data into classes. This allows
us to avoid passing the same objects through many functions that don't directly
use them. Now, functions that need to access data can do so from the class
state.

Reviewed By: jhenderson, smeenai

Differential Revision: https://reviews.llvm.org/D113127