<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/87694>87694</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Backport unreachable fix for C++, regressed in LLVM 18
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

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

    <tr>
      <th>Reporter</th>
      <td>
          ian-twilightcoder
      </td>
    </tr>
</table>

<pre>
    We should backport https://github.com/llvm/llvm-project/pull/86748 because it fixes a conflicting `unreachable` macro in C++ in some situations, which were introduced in clang in LLVM 18.

/cherry-pick df69a305253f1d1b4a4066055a07101a4cc03e55
</pre>
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