We need special handling for protocols whose requirement
signature exists but is in a serialized state, as we
cannot run the StructuralRequirementsRequest on such
a protocol as there's no work to be done, effectively.
- Revised "Substitution Maps" chapter:
- New "Subclassing" section
- New "SIL Type Lowering" section
- New "Opaque Result Types" chapter
- Various smaller edits
New PDF will be available shortly at https://download.swift.org/docs/assets/generics.pdf.
