[llvm] [Utils][update_mc_test_checks] Support updating round-trip tests. (PR #164425)

[Ivan Kosarev via llvm-commits]

kosarev wrote:

> > I would think the ultimate way is to let the update script understand the lit.cfg file and thus it can parse and generate the same run line as llvm-lit does. I think the current approach is also acceptable.
> 
> This would be great and I actually started on working on this a while back: importing lit and having lit do all the parsing and substitutions. I should try to resurrect that change. The only downside is that you now need give the script valid build directory to get all the right substitutions, but that should hopefully be an acceptable tradeoff.

I'd see this an improvement as it would unify the CLI amongst the update scripts; you just specify the build directory, and that's it. The tests themselves have lots of dependencies, such as FileCheck itself, `not`, etc. anyway.

We can still have options to specify individual tools when needed.

> I feel this feature a little hidden in the script. Should we add a help option to list all the supported substitution and what it is trying to do?

Not sure about that. The update scripts handling substitutions looks more the default expectation rather than a feature -- and the lack of it, therefore, is to be considered a bug.

https://github.com/llvm/llvm-project/pull/164425