title: Code completion not working in Clion 2019.1 for c++2a
body: <p>I have Clion 2019.1 and gcc-9.0.1(trunk). Code completion and syntax analyser don't recognize the <code>concept</code> keyword and other c++2a features released in gcc-9.0.1.</p>
  
  <p>In <code>CmakeLists.txt</code> i use <code>CMAKE_CXX_STANDARD 20</code> and compilers flags <code>-fconcept -std=c++2a</code><br>
  The code compiles fine.</p>