As a developer of Qt, I expect QDoc to parse C++ as if all __cpp_ feature macros were defined to infinity

When writing C++-feature dependent API, I don't want to uglify already-ugly code like

#ifdef__cpp_char8_t
    Q_WEAK_OVERLOAD
    QString fromUtf8(const char8_t *s)
    { return fromUtf8(reinterpret_cast<const char*>(s)); }
#endif

by having to think about what this means for QDoc and thus ending up with

#if defined(__cpp_char8_t) || defined(Q_QDOC)

QDoc just has to parse C++ feature macros as defined (bonus: even unknown ones) and to inifinity (so all version checks will be true, too).

This should also work in 5.15, because when C++20 hits the road, we'll be having a lot of these in our code.