Reland "[llvm] llvm-tapi-diff"

This is relanding commit d1d36f7a .
This patch additionally addresses failures found in buildbots & post review comments.

This patch introduces a new tool, llvm-tapi-diff, that compares and returns the diff of two TBD files.

Reviewed By: ributzka, JDevlieghere

Differential Revision: https://reviews.llvm.org/D101835