More helpful error message when instantiating an abstract class

Calling a literal ActiveRecord::Base.new raises NoMethodError,
since it ends up calling Class.abstract_class? which does not exist.
Similarly, instantiating an actual abstract class hits the database,
when conventionally it should immediately throw NotImplementedError.

ActiveRecord::Base can't be made abstract without breaking many,
many things, so check for it separately.
Merge pull request #9474 from HonoreDB/master

More helpful error message when instantiating an abstract class

Conflicts:
	activerecord/CHANGELOG.md