code: async function getClangVersion () {
    try {
      await fs.which('clang');
    } catch (e) {
      log.info('Cannot find clang executable on the local system. ' +
        'Are Xcode Command Line Tools installed?');
      return null;
    }
    const {stdout} = await exec('clang', ['--version']);
    const match = /clang-([0-9.]+)/.exec(stdout);
    if (!match) {
      log.info(`Cannot parse clang version from ${stdout}`);
      return null;
    }
    return match[1];
  }
docs: Check https://trac.macports.org/wiki/XcodeVersionInfo
  to see the actual mapping between clang and other components.
  
  @returns {?string} The actual Clang version in x.x.x.x or x.x.x format,
  which is supplied with Command Line Tools. `null` is returned
  if CLT are not installed.