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

[Henrik G. Olsson via cfe-commits]

hnrklssn wrote:

> 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

Makes sense, I'll separate the changes into 3 PRs.

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