C++: Use soft asserts instead of assert()

Some qmake versions ignore "DEFINES += NDEBUG" on Windows.

Change-Id: Ibdf3b1036c2331dcef61dcd278463c02754044d2
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>

src/libs/3rdparty/cplusplus/AST.h

@@ -47,7 +47,7 @@ public:
         if (value)
             return value->firstToken();
 
-        // ### assert(0);
+        // ### CPP_CHECK(0);
         return 0;
     }
 
@@ -58,7 +58,7 @@ public:
         if (lv)
             return lv->lastToken();
 
-        // ### assert(0);
+        // ### CPP_CHECK(0);
         return 0;
     }