[llvm] Reapply "[lit] Implement builtin umask (#94621)" (PR #155850)

[Tex Riddell via llvm-commits]

tex3d wrote:

@boomanaiden154

This change breaks on Windows pipelines because the error output is: `'umask' not supported on this system` instead of what's expected for the error tests.

It seems like the top-level `shtest-umask.py` should be the one with `# UNSUPPORTED: system-windows`, doesn't it?

https://github.com/llvm/llvm-project/pull/155850