Method setFlag

tests/catch.h:4063–4065

                convertInto( stringValue, p.*member );
            }
            virtual void setFlag( C& p ) const {
                convertInto( true, p.*member );
            }
            virtual bool takesArg() const { return !IsBool<M>::value; }
            virtual IArgFunction<C>* clone() const { return new BoundDataMember( *this ); }
            M C::* member;