Merge pull request #489 from apple/ewilde/5.10-guard-parse-version

[5.10] Guard `parse_version`

Tests/Functional/lit.cfg

@@ -143,10 +143,14 @@ config.substitutions.append(('%{xctest_checker}', '%%{python} %s' % xctest_check
 config.substitutions.append( ('%{python}', pipes.quote(sys.executable)) )
 
 # Conditionally report the Swift 5.5 Concurrency runtime as available depending on the OS and version.
+# Darwin is the only platform where this is a limitation.
 (run_os, run_vers) = config.os_info
-os_is_not_macOS = run_os != 'Darwin'
-macOS_version_is_recent_enough = parse_version(run_vers) >= parse_version('12.0')
-if os_is_not_macOS or macOS_version_is_recent_enough:
+if run_os == 'Darwin':
+    assert run_vers != "", "No runtime version set."
+    if parse_version(run_vers) >= parse_version('12.0'):
+        config.available_features.add('concurrency_runtime')
+else:
+    # Non-Darwin platforms have a concurrency runtime
     config.available_features.add('concurrency_runtime')
 if run_os == 'Windows':
     config.available_features.add('OS=windows')