Introduce ProtocolConformanceRef. NFC.

The main idea here is that we really, really want to be
able to recover the protocol requirement of a conformance
reference even if it's abstract due to the conforming type
being abstract (e.g. an archetype).  I've made the conversion
from ProtocolConformance* explicit to discourage casual
contamination of the Ref with a null value.

As part of this change, always make conformance arrays in
Substitutions fully parallel to the requirements, as opposed
to occasionally being empty when the conformances are abstract.

As another part of this, I've tried to proactively fix
prospective bugs with partially-concrete conformances, which I
believe can happen with concretely-bound archetypes.

In addition to just giving us stronger invariants, this is
progress towards the removal of the archetype from Substitution.

include/swift/Serialization/ModuleFile.h

@@ -595,7 +595,7 @@ public:
 
   virtual void
   loadAllConformances(const Decl *D, uint64_t contextData,
-                      SmallVectorImpl<ProtocolConformance*> &Conforms) override;
+                    SmallVectorImpl<ProtocolConformance*> &Conforms) override;
 
   virtual TypeLoc loadAssociatedTypeDefault(const AssociatedTypeDecl *ATD,
                                             uint64_t contextData) override;
@@ -653,9 +653,7 @@ public:
   Optional<Substitution> maybeReadSubstitution(llvm::BitstreamCursor &Cursor);
 
   /// Recursively reads a protocol conformance from the given cursor.
-  ///
-  /// Note that a null conformance is valid for archetypes.
-  ProtocolConformance *readConformance(llvm::BitstreamCursor &Cursor);
+  ProtocolConformanceRef readConformance(llvm::BitstreamCursor &Cursor);
 
   /// Read the given normal conformance from the current module file.
   NormalProtocolConformance *