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

[Paul T Robinson via llvm-commits]

pogo59 wrote:

> It's still up to anyone who writes a test using umask to add something like UNSUPPORTED: system-windows.

That's not friendly to test writers IMO. If we know it won't work, we might as well just fail saying so up front, and save people having to research and diagnose mystery failures on Windows when they don't have Windows locally to try it on.

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