title: How to read llvm-cov json format?
body: <p>I'm able to export code coverage data by llvm-cov in json format, but the content seems mysterious to me. What does each number in <code>segments</code> section mean?</p>
  
  <pre><code>{
     "filename":"file.m",
     "segments":[
        [
           11,
           22,
           23,
           1,
           1
        ],
        [
           12,
           11,
           23,
           1,
           1
        ],
        ...
     ],
     "expansions":[
  
     ],
     "summary":{
        ...
     }
  }
  </code></pre>