Disable tab expansion when counting the columns in block comments.

To fully support this, we also need to expand tabs in the text before
the block comment. This patch breaks indentation when there was a
non-standard mixture of spaces and tabs used for indentation, but
fixes a regression in the simple case:
{
  /*
   * Comment.
   */
  int i;
}
Is now formatted correctly, if there were tabs used for indentation
before.