Fixes #1249

Running RDOC_USE_PRISM_PARSER=1 make html in ruby/ruby fails with 'RDoc::ClassModule#superclass': stack level too deep (SystemStackError).
Parsing the OpenSSL::Cipher::Cipher in the code below succeeds, but with recursive superclass.

class OpenSSL
  class Cipher
    # Defines class OpenSSL::Cipher::Cipher with superclass=OpenSSL::Cipher
    # But PrismRuby parses it as class OpenSSL::Cipher::Cipher with superclass=OpenSSL::Cipher::Cipher
    class Cipher < Cipher; end
  end
end

In class definition, superclass part is evaluated before constant definition.
PrismRuby should do the same: resolve superclass first, then add class with the given classname.