[llvm] [LLVM][lit] add system-cygwin feature (PR #152780)

[via llvm-commits]

https://github.com/jeremyd2019 created https://github.com/llvm/llvm-project/pull/152780

There are a few tests in clang that gate on system-windows but also don't work on Cygwin, so add a system-cygwin feature for them to use.

>From 17831bb905b87ead92902db63b72759b378e8967 Mon Sep 17 00:00:00 2001
From: Jeremy Drake <github at jdrake.com>
Date: Fri, 8 Aug 2025 12:28:35 -0700
Subject: [PATCH] [LLVM][lit] add system-cygwin feature

There are a few tests in clang that gate on system-windows but also
don't work on Cygwin, so add a system-cygwin feature for them to use.
---
 llvm/utils/lit/lit/llvm/config.py | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/llvm/utils/lit/lit/llvm/config.py b/llvm/utils/lit/lit/llvm/config.py
index 649636d4bcf4c..b04fb25f3c030 100644
--- a/llvm/utils/lit/lit/llvm/config.py
+++ b/llvm/utils/lit/lit/llvm/config.py
@@ -107,6 +107,8 @@ def __init__(self, lit_config, config):
             features.add("system-solaris")
         elif platform.system() == "OS/390":
             features.add("system-zos")
+        elif sys.platform == "cygwin":
+            features.add("system-cygwin")
 
         # Native compilation: host arch == default triple arch
         # Both of these values should probably be in every site config (e.g. as