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

[Jay Foad via llvm-commits]

jayfoad wrote:

> No, the umask implementation should say something like `if onWindows: die("umask doesn't work on Windows")` Not that I know how to check for Windows or how to spell `die` in Pythonic. But you get the idea.

I see. Done.

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