Merge pull request #495 from jmschonfeld/support-swiftfoundation-6-0-cherrypick

[6.0] Add new include path into Foundation when building tests

Author: parkera

Tests/Functional/lit.cfg

@@ -99,6 +99,7 @@ else:
         '-L', os.path.join(foundation_dir, 'lib'),
         '-I', foundation_dir,
         '-I', os.path.join(foundation_dir, 'swift'),
+        '-I', os.path.join(foundation_dir, '_CModulesForClients'),
         '-Xcc', '-F', '-Xcc', foundation_dir,
     ])