[clang] [llvm] [Utils] Add new --update-tests flag to llvm-lit (PR #108425)

[Henrik G. Olsson via cfe-commits]

hnrklssn wrote:

> Thank you. This pretty much LGTM, but it just occurred to me that there's a docs/CommandGuide/lit.rst which should be updated to document the new option.

Done

> As update-verify-tests was reverted, can you rebase this to just the update_test_checks support? On the LLVM side, we definitely want to have this feature.

and done

https://github.com/llvm/llvm-project/pull/108425