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

[Jay Foad via llvm-commits]

jayfoad wrote:

> You probably want to add something within lit's own test suite to exercise the umask function.

Done.

> Might also want to show it fails cleanly on Windows.

My umask implementation in lit just builds on the "umask" argument to Python's subprocess.Popen. I doubt that has any useful effect on Windows, but I don't explicitly check for that. It's still up to anyone who writes a test using umask to add something like `UNSUPPORTED: system-windows`.

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