ASTVisitor: Add QBS_CHECKS for things we believe cannot happen.
ASTVisitor: Add QBS_CHECKS for things we believe cannot happen.

Change-Id: I39be6858bd9fa83eb64f72cd82ed274a025170f5
Reviewed-by: Joerg Bornemann <47777b9406dbddd02a3fdca8341789a013805a11@theqtcompany.com>

gatzka/qt-labs-qbs,qt-labs/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qt-labs/qbs,gatzka/qt-labs-qbs,gatzka/qt-labs-qbs,qt-labs/qbs,gatzka/qt-labs-qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,gatzka/qt-labs-qbs,gatzka/qt-labs-qbs,qtproject/qt-labs-qbs,gatzka/qt-labs-qbs,qtproject/qt-labs-qbs