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

[Nicolai Hähnle via cfe-commits]

https://github.com/nhaehnle commented:

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.

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