An "abstract" ProtocolConformanceRef is a conformance of a type
parameter or archetype to a given protocol. Previously, we would only
store the protocol requirement itself---but not track the actual
conforming type, requiring clients of ProtocolConformanceRef to keep
track of this information separately.
Record the conforming type as part of an abstract ProtocolConformanceRef,
so that clients will be able to recover it later. This is handled by a uniqued
AbstractConformance structure, so that ProtocolConformanceRef itself stays one
pointer.
There remain a small number of places where we create an abstract
ProtocolConformanceRef with a null type. We'll want to chip away at
those and establish some stronger invariants on the abstract conformance
in the future.
