Impact of C++20 constexpr on our code

C++20 added futher relaxations for constexpr functions:

1. virtual function calls
2. dynamic_cast and polymorphic typeid
3. try-catch blocks
4. asm, unless "executed" (__cpp_constexpr >= 201907L)
5. T t; (default initialisation) (__cpp_constexpr >= 201907L)
6. changing the active member of a union (__cpp_constexpr >= 202002L)
7. memory allocation and deallocation ("constexpr std::string and std::vector") (defined(__cpp_constexpr_dynamic_alloc))

As these are all not backwards-compatible with C++17, so all we can realistically do is try and make some interesting functions or classes C++20 constexpr, much like we did in C++11 and C++14 (Q_RELAXED_CONSTEXPR).

For that, we might want to add a Q_CPP20_CONSTEXPR, though the extensions enumerated above have been merged into the IS in different meetings, so the __cpp_constexpr macro has several potential values on it's way to full C++20 conformance, so one macro may not be sufficient, or we need to use __cpp_constexpr manually (at least we wouldn't need to check for it to be defined anymore, one might hope, simplifying the condition to #if __cpp_constexpr >= 201907L).