Guard parse_version

The most recent versions of `parse_version` throw an exception if the
version is empty. The version passed in is only set on Darwin (call to
`mac_ver()`, so it's causing test failures on newer versions of Linux
since the test suite can't even start.

Now, the only reason for the version parse is because the tests are
looking at whether or not concurrency is available on the OS. This is
only a limitation if we're working with Darwin. Swift 5.10 on Windows
and Linux always have a Swift 5.10 concurrency runtime, so we don't even
need to check for a version.

rdar://128502662

Author: etcwilde

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')