[llvm] [lit] Implement builtin umask (PR #94621)

[Jay Foad via llvm-commits]

jayfoad wrote:

> Now, ninja check-lit already has another 5 failures in my Windows environment, so I'd be okay with it if you want to file an issue (assign to me) to fix up this case too. I'm annoyed enough at the situation to devote some time to tidying up the check-lit failures, and once those are actually working I can file an issue to get check-lit added to the bots.

Thanks! It's pretty painful for me to work on Windows failures that I can't reproduce locally, so I will happily take you up on this offer.

Just to confirm: are you happy for me to commit this patch as-is?

https://github.com/llvm/llvm-project/pull/94621