title: Clang: write a function's AST from original file to a new file
body: <p>I'm a novice for Clang who is trying to analyze AST via libtooling.
  I want to find a particular function, and move its AST from original source file to a new file.</p>
  
  <p>I've known how to find the function by MatchFinder. 
  Now, I was wondering how to write its AST to a new file(.c or .cpp)</p>
  
  <p>Thanks in advance!  </p>