[llvm] [Code Coverage] Add a tool to check test coverage of a patch (PR #71841)

[Tobias Hieta via llvm-commits]

tru wrote:

I know we are not doing this right now, but I wonder if we should have tests for python scripts as well so that we can run it with our supported python version and see if it works, because it can be hard to spot in a review.

I will make a sweep on the code later today.

https://github.com/llvm/llvm-project/pull/71841