title: EFCore 3 GroupBy+ToDictionary raises client side GroupBy is not supported
body: <p>The following <code>GroupBy</code> query cannot be translated and will raise an error: <code>client side GroupBy is not supported</code></p>
  
  <pre><code>IEnumerable&lt;int&gt; ids = new List&lt;int&gt; { 1, 2, 3 };
  var q = db.Comments.Where(x =&gt; ids.Contains(x.Identifier))
      .GroupBy(x =&gt; x.Identifier);
  var map = await q.ToDictionaryAsync(x =&gt; x.Key, x =&gt; x.Count());
  </code></pre>
  
  <p>How should the query be rearranged to be able to run on the dbms?</p>