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

[Nicolai Hähnle via llvm-commits]

nhaehnle wrote:

As for the question about whether to default this: I do think it should be the default, certainly in the long run. It could benefit from a bit more testing, but on the other hand the `--reset-variable-names` command-line option is a good way to bypass most of the new logic and can serve as a temporary workaround if a problem is indeed found.

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