[llvm][tapi-diff] Transition diff-tool to be more general purpose

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