Merge pull request swiftlang#28098 from brentdax/a-link-to-the-past-5.1

[5.1] Fix flaky Driver tests

Author: beccadax

test/lit.cfg

@@ -1422,9 +1422,17 @@ if hasattr(config, 'target_cc_options'):
 else:
     config.substitutions.append(('%target-cc-options', ''))
 
-config.substitutions.append(
-    (r'%hardlink-or-copy\(from: *(.*), *to: *(.*)\)',
-     SubstituteCaptures(r'ln \1 \2 || cp \1 \2')))
+# WORKAROUND(rdar://53507844): On some macOS versions, we see flaky failures in
+# tests which create a hard link to an executable and immediately invoke it.
+# Work around this by always copying on Darwin.
+if platform.system() == 'Darwin':
+  config.substitutions.append(
+      (r'%hardlink-or-copy\(from: *(.*), *to: *(.*)\)',
+       SubstituteCaptures(r'cp \1 \2')))
+else:
+  config.substitutions.append(
+      (r'%hardlink-or-copy\(from: *(.*), *to: *(.*)\)',
+       SubstituteCaptures(r'ln \1 \2 || cp \1 \2')))
 
 config.substitutions.append(('%utils', config.swift_utils))
 config.substitutions.append(('%line-directive', '%r %s' % (sys.executable, config.line_directive)))