They can't compile the new constexpr decomposer because
`ITransientExpression` has a defaulted virtual destructor. We could
instead drop the virtual destructor - it only exists to silence
static analysis tools complaining about having virtual functions
without virtual destructor - but GCC 5 and 6 are positively ancient,
so we drop them instead.
Martin Hořeňovský
bbba3d8a06