title: Swift date(byAdding:to:) returns nil for trivial calculation in REPL
body: <p>I expect this to return a <code>Date</code> object representing the time one hour from now:</p>
  
  <pre class="lang-swift prettyprint-override"><code>Calendar.current.date(byAdding: DateComponents(hour: 1), to: Date()) 
  </code></pre>
  
  <p>However it returns <code>nil</code>. This seems like a rather straightforward use of the API so perhaps I have some fundamental misconception about how this API should work?</p>
  
  <p>This <em>only</em> appears to affect the <code>swift</code> command line REPL included with Xcode (version 11.3.1 on macOS Catalina 10.15.3).</p>
  
  <p>Interestingly, wrapping the above code in a <code>print()</code> call forces it to print the correct result. So why is it showing <code>nil</code>? </p>
  
  <pre><code>Apple Swift version 5.1.3 (swiftlang-1100.0.282.1 clang-1100.0.33.15)
  Target: x86_64-apple-darwin19.3.0
  </code></pre>