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

[Luke Lau via cfe-commits]

lukel97 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?
> > 
> > 
> > Just normal tests that passed. Is this intentional?
> 
> Definitely not. I'll take a look.

Thanks! If it's of any help, here's one test where running`./build/bin/llvm-lit --update-tests llvm/test/Transforms/LoopVectorize/AArch64/conditional-branches-cost.ll` seems to cause a spurious update on my end.

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