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

[Paul T Robinson via llvm-commits]

pogo59 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.

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