title: Strange clangd crash when using \n instead of endl with -std=c++2a
body: <p>This is the reference source file</p>
  <pre class="lang-cpp prettyprint-override"><code>#include &lt;iostream&gt;
  int main() {
      int x = 7;
      std::cout &lt;&lt; x &lt;&lt; '\n';
  }
  </code></pre>
  <p>If I open a line <em><strong>before</strong></em> the line with <code>cout</code> and write <code>std::</code> I correctly see YCM's semantic cmpletion popup; if I do the same <em><strong>after</strong></em> the line with <code>cout</code>, I get a <code>ResponseAbortedException: Response Aborted</code> error.</p>
  <p><code>:YcmDebugInfo</code> gives</p>
  <pre class="lang-none prettyprint-override"><code>Printing YouCompleteMe debug information...
  -- Resolve completions: Up front
  -- Client logfile: /tmp/ycm_fiumsvcv.log
  -- Server Python interpreter: /usr/bin/python3
  -- Server Python version: 3.8.6
  -- Server has Clang support compiled in: True
  -- Clang version: clang version 10.0.0 (https://github.com/ycm-core/llvm 038587147cf2f97d1c3e677042f69560c65b5bea)
  -- Extra configuration file found and loaded
  -- Extra configuration path: /home/enrico/.ycm_extra_conf.py
  -- C-family completer debug information:
  --   Clangd not running
  --   Clangd executable: ['/home/enrico/.vim/plugged/YouCompleteMe/third_party/ycmd/third_party/clangd/output/bin/clangd', '-cross-file-rename', '-header-insertion-decorators=0', '-resource-dir=/home/enrico/.vim/plugged/YouCom
  pleteMe/third_party/ycmd/third_party/clang/lib/clang/10.0.0', '-limit-results=500']
  --   Clangd logfiles:
  --     /tmp/clangd_stderryqofwzop.log
  --   Clangd Server State: Dead
  --   Clangd Project Directory: /home/enrico
  --   Clangd Settings: {}
  --   Clangd Compilation Command: ['clang-tool', '-x', 'c++', '-std=c++2a', '-Wall', '-Wextra', '-Werror', '/home/enrico/deleteme.cpp']
  -- Server running at: http://127.0.0.1:42807
  -- Server process ID: 25037
  -- Server logfiles:
  --   /tmp/ycmd_42807_stdout_9pmpk9zp.log
  --   /tmp/ycmd_42807_stderr_iat9y0gn.log
  </code></pre>
  <p><a href="https://asciinema.org/a/0hRAvkePSMq3Ht60epsiCuZ7W" rel="nofollow noreferrer">This is a screencast</a> of the issue.</p>