Fix #13970 cfg/qt.cfg - add definition for QDate::isValid()

[winterz]

Add the bool isValid() signature.
bool isValid(int, int, int) is already there.

[chrchr-github]

Does this solve an issue for you? The existing definition already claims to handle both overloads via the default arguments.

[winterz]

with the current definition the following code snippet gives a false positive assertWithSideEffect

int IncidenceRecurrence::weekdayCountForMonth(const QDate &date) const
{
    /* cppcheck-suppress assertWithSideEffect */
    Q_ASSERT(date.isValid());

QDate::isValid() doesn't muck with date so it shouldn't cause any side effects, right?

[chrchr-github]

Then you should add <const/> to the existing definition, and a test case in qt.cpp.

[chrchr-github]

Side note: The attribute <const/> indicates a constant function, not just member-constness, which seems to be correct in this case. But we might not always interpret this consistently as far as I remember. See #6385