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

[Paul T Robinson via llvm-commits]

================
@@ -4,7 +4,6 @@
 ## Windows has no umask so this test makes no sense, nor would
 ## it work because there is no umask(1) in a Windows environment
----------------
pogo59 wrote:

Probably it was. But that's not relevant anymore now that the internal shell has umask.

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