Any county having more than 85,000 inhabitants, according to the census of the United States, may frame and adopt and amend a charter for its own government as provided in this Article, and upon such adoption shall be a body corporate and politic. In addition and as an alternative to the foregoing, any county which attains first class county status and maintains such status for at least two years shall be authorized to frame and adopt and amend a charter for its own government as provided by this Article, and upon such adoption by a vote of the qualified electors of such county shall be a body corporate and politic. Counties which adopt or which have adopted a charter or constitutional form of government shall be a separate class of counties outside of the classification system established under § 8 of this Article.