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

[Henrik G. Olsson via cfe-commits]

hnrklssn wrote:

> Thanks for relanding this, it's super useful. It seems to rerun update_test_[checks.py](http://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?

Are these XFAIL tests, or just normal tests that passed?

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