[llvm] update_test_checks: keep meta variables stable by default (PR #76748)

[Nicolai Hähnle via llvm-commits]

nhaehnle wrote:

This PR is now ready for another round of reviews.

I've done a bunch more testing with real-world examples and ended up changing the high-level algorithmic approach somewhat to get better results. As a bonus, the core diffing algorithm is now in a separate function.

https://github.com/llvm/llvm-project/pull/76748