[K2] Missed `override` modifier for generic functions

[vmishenev]

class A<T>  : C<T>()

open class C<T> : D<T> {
    override fun f(a: T) = 1
}

interface D<T> {
    fun f(a: T) = 0
}

The signature of A.f does not have override modifier. K1:

K2:

Related #3677