Specification: Update class implementation completeness check

[eernstg]

Cf. this comment, it is not correct to inherit an implementation whose parameters are not covariant into a class where the same declaration would only be correct when it is taken into account that the parameters would be covariant-by-declaration implicitly. But it is allowed to inherit an implementation whose parameters are not covariant into a class where the same declaration would only be correct when it is taken into account that the parameters would be covariant-by-class.

This needs to be taken into account in the specification of what it means for a class to be fully implemented. We already have this:

It is a compile-time error if C has a concrete accessible member with the same name
as m, with a method signature m₀ which is not a correct override
of m (11.2.2), unless that concrete member is a noSuchMethod forwarder (10.1.2).

We need to make sure that the inherited member, with declaration D, whose signature is m₀, is not considered to have parameters that are covariant-by-declaration based on C, it must be computed based on the enclosing class of the declaration D.

[eernstg]

Done in #1770.