llvm-project/clang/test/Syntax/lit.local.cfg

cxx_bnf_file = os.path.join(config.clang_src_dir, 'lib', 'Tooling', 'Syntax',
                            'Pseudo', 'cxx.bnf')
config.substitutions.append(('%cxx-bnf-file',
                             '%s' % (cxx_bnf_file)))