[clang] [llvm] Reland "[Utils] Add new --update-tests flag to llvm-lit" (PR #153821)

[Luke Lau via llvm-commits]

lukel97 wrote:

Thanks for relanding this, it's super useful. It seems to rerun update_test_checks.py on tests that weren't failing, which can cause some spurious diffs that are just UTC regens. Is it possible to restrict this to only update the tests that failed?

https://github.com/llvm/llvm-project/pull/153821