Report on C++ visibility and other attributes

[adetaylor]

This change reports extra C++ information about items:

• Whether methods are virtual or pure virtual or neither
• Whether a method is a "special member", e.g. a move constructor
• Whether a method is defaulted or deleted
• C++ visibility (for structs, enums, unions and methods)

It builds on top of #3145.

Once #3139 is merged, a follow up PR should enhance the test to cover those cases too.

Part of https://github.com/google/autocxx/issues/124