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

[Paul T Robinson via llvm-commits]

================
@@ -0,0 +1,5 @@
+import lit.formats
+
+config.name = "shtest-umask"
+config.suffixes = [".txt"]
+config.test_format = lit.formats.ShTest(execute_external=False)
----------------
pogo59 wrote:

You'll need to add this:
```
if sys.platform.startswith("win") or sys.platform.startswith("cygwin"):
    config.available_features.add("system-windows")
```
because this sandboxed configuration doesn't run the parent lit.cfg. But with that, and changing the expectation in shtest-umask.py to `Unsupported`, it should work. (A hacked-up version does work in my Windows environment.)

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