Merge allows a user to merge different files (tbds for now or dylibs in
the future) to emit out a single tbd with all input contents. This does
require that all inputs represent the same library.
This is the first of transition tapi-diff to be readtapi.
This tool will eventually replace functionality for Xcode's `xcrun tapi stubify` and
`xcrun tapi archive`.
This patch updates the tool name and is a minor refactor for the driver to handle more options.
Reviewed By: zixuw
Differential Revision: https://reviews.llvm.org/D153045