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

[Jay Foad via llvm-commits]

jayfoad 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.
> 
> I don't understand what kind of diagnostic you want. If I'm on Linux and I write a test that uses umask, I do not want it to fail with "error: this would not work on Windows".

Also note I'm not changing the status quo in this regard. People already write tests that use umask, and don't immediately find out that they won't work on Windows.

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