[darwin] Set 'darwin' feature to fix tests using UNSUPPORTED/REQUIRES

This should ultimately be upstreamed, but it depends on other changes to
lit that have not been upstreamed yet (rdar://26780128).

rdar://56817666

Author: benlangmuir

clang/test/lit.cfg.py

@@ -155,6 +155,9 @@ def is_filesystem_case_insensitive():
 if platform.system() not in ['Windows']:
     config.available_features.add('can-remove-opened-file')
 
+# *-apple-macosx should also be XFAILED when 'darwin' is XFAILED.
+if lit.util.isMacOSTriple(config.target_triple):
+   config.available_features.add('darwin')
 
 def calculate_arch_features(arch_string):
     features = []