Emit copies of default implementations in protocol extensions and superclass declarations in conforming types and subclasses respectively using a virtual USR, i.e. `${REAL_USR}::SYNTHESIZED::${CONFORMING_OR_SUBCLASS_TYPE_USR}`.
- Add a -skip-synthesized-members option to skip these synthesized members.
- Create a new wrapping `Symbol` type that can also contain a base type declaration as well as the inherited declaration for those synthesized cases. Move some symbol-specific APIs there.
- Doc comments can “cascade” down to protocol extensions or refinements in concrete types. When emitting the doc comment for a symbol, look up through to superclasses or protocol requirements for where a doc comment is actually written.
- Clean up filtering of implicitly private (e.g. “public underscored”) types
rdar://problem/59128787
