[clang] [llvm] [Utils] add --update-tests flag to llvm-lit (PR #97369)

[Nicolai Hähnle via cfe-commits]

https://github.com/nhaehnle commented:

I think the integration into lit is okay. The diff updater seems neat, but I think it could be taken into a separate change, see also the inline comment. In general, I think this PR really does three separate things, that ought to be in separate changes:

* Add update-verify-tests.py
* Add a way to update tests from within llvm-lit
* Add the diff-test-updater to llvm-lit

https://github.com/llvm/llvm-project/pull/97369