Add a macro for C++20 constexpr

Update: For now, there is only one single use of this macro: explicit destructors can be constexpr. There may be more once we increase our usage of C++20. Therefore, there is no need to add it now. Instead we are adding Q_DECL_CONSTEXPR_DTOR.

_____________________________________________________________________________________

Some changes and enhancements were made to the constexpr feature in C++20 making it more flexible.  For example: It allows now dynamic memory allocation and deallocation, constexpr explicit destructors, ect.  https://en.cppreference.com/w/cpp/language/constexpr#:~:text=Constexpr%20lambda-,201907L,-(C%2B%2B20)}}

Create a macro for C++20 constexpr instead of writing multiple times:

#if __cpp_constexpr >= 201907L

    constexpr

#endif 

I suggest a simple macro CONSTEXPR_20 in qcompilerdetection.h:

#if __cpp_constexpr >= 201907L

    define CONSTEXPR_20     constexpr
#else

   define CONSTEXPR_20
#endif