Fix TopLevelVisitor adding existing ClassDef type to current scope

[straight-shoota]

When a type is reopened, the compiler adds it as nested type to the current scope (i.e. where it's reopened). That's wrong because the reopened type is already scoped to the location of the original location and this could lead to cycles.

For example, the following program would crash the compiler with an infinite recursion:

class Foo
  alias Bar = Foo

  class Bar
  end
end

This fix makes sure to only add newly defined types to the current scope, not reopened ones. All other visitor methods which defined types already do this, only ClassDef was unconditionally adding to the current scope.