Note the need for --param=build_config=whatever in the Hacking Clang document, from Yuri Gribov

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@124130 91177308-0d34-0410-b5e6-96231b3b80d8

www/hacking.html

@@ -185,6 +185,10 @@
  (path to llvm)/llvm/tools/clang/test
   </tt>
 
+  <p>For CMake builds e.g. on Windows with Visual Studio, you will need
+  to specify your build configuration (Debug, Release, etc.) via
+  <tt>--param=build_config=(build config)</tt>.</p>
+
   <p>To run a single test:</p>
   
   <tt>