Consider implementing clazy warning for implicit construction of QVariant(bool)

Given a function returning QVariant, if you accidentally `return false` it will silently construct and return a QVariant containing a bool with the value `false`. In modern C++ this could be written as `return

` if it was actually intentional.

Having a warning + fixit in tooling like Clazy would make this easily discoverable.
As a bonus, detect implicit conversion from types with implicit conversion to bool, like smart pointers (see QTBUG-43358).