[LIT][Cygwin] Mark ulimit -n unsupported for Cygwin

Cygwin doesn't support `ulimit -n` because Windows doesn't provide such functionality.

Author: kikairoya

llvm/utils/lit/lit/run.py

@@ -137,6 +137,6 @@ def _increase_process_limit(self):
                     "Raised process limit from %d to %d" % (soft_limit, desired_limit)
                 )
         except Exception as ex:
-            # Warn, unless this is Windows or z/OS, in which case this is expected.
-            if os.name != "nt" and platform.system() != "OS/390":
+            # Warn, unless this is Windows, z/OS, or Cygwin in which case this is expected.
+            if os.name != "nt" and platform.system() != "OS/390" and platform.sys.platform != "cygwin":
                 self.lit_config.warning("Failed to raise process limit: %s" % ex)

llvm/utils/lit/tests/shtest-ulimit.py

@@ -3,7 +3,7 @@
 # ulimit does not work on non-POSIX platforms.
 # Solaris for some reason does not respect ulimit -n, so mark it unsupported
 # as well.
-# UNSUPPORTED: system-windows, system-solaris
+# UNSUPPORTED: system-windows, system-cygwin, system-solaris
 
 # RUN: not %{lit} -a -v %{inputs}/shtest-ulimit --order=lexical | FileCheck %s