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

[Nikita Popov via llvm-commits]

nikic wrote:

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.

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