<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/55551>55551</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [doc] Update document for casting infrastructure
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            documentation
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          0x59616e
      </td>
    </tr>
</table>

<pre>
    Casting infrastructure has been updated by [D123901](https://reviews.llvm.org/D123901), but the [document](https://llvm.org/docs/HowToSetUpLLVMStyleRTTI.html) is still behind. We need to update it to  reflect the changes.

cc @bzcheeseman 
</pre>
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