[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.

Author: jeremyd2019

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