Add (partial) support for Objective-C category records in ExtractAPI.
The current ExtractAPI collects everything for an Objective-C category,
but not fully serialized in the SymbolGraphSerializer. Categories
extending external interfaces are disgarded during serialization, and
categories extending known interfaces are merged (all members surfaced)
into the interfaces.
Differential Revision: https://reviews.llvm.org/D122774
178aad9b94