function _processPlaceholderMarkdownLink(icu) {
  const message = icu.message;

  // Check for markdown link common errors, ex:
  // * [extra] (space between brackets and parens)
  if (message.match(/\[.*\] \(.*\)/)) {
    throw Error(`Bad Link spacing in message "${message}"`);
  }
  // * [](empty link text)
  if (message.match(/\[\]\(.*\)/)) {
    throw Error(`markdown link text missing in message "${message}"`);
  }

  icu.message = '';
  let idx = 0;

  for (const segment of Util.splitMarkdownLink(message)) {
    if (!segment.isLink) {
      // Plain text segment.
      icu.message += segment.text;
      continue;
    }

    // Otherwise, append any links found.
    const startPlaceholder = `LINK_START_${idx}`;
    const endPlaceholder = `LINK_END_${idx}`;
    icu.message += '$' + startPlaceholder + '$' + segment.text + '$' + endPlaceholder + '$';
    idx++;
    icu.placeholders[startPlaceholder] = {
      content: '[',
    };
    icu.placeholders[endPlaceholder] = {
      content: `](${segment.linkHref})`,
    };
  }
}
Convert markdown html links into placeholders.

@param {IncrementalCtc} icu