Serialization: Serialize ProtocolDecl::getSuperclassDecl() instead of ProtocolDecl::getSuperclass()

Protocols with a superclass bound written as `protocol P where Self: C`
return null from getSuperclass(). Unqualified lookup only cares about
getSuperclassDecl(), so serialize that instead.

Fixes rdar://problem/124478687.

lib/Serialization/Serialization.cpp

@@ -4353,7 +4353,7 @@ public:
                                  ->requiresClass(),
                                proto->isObjC(),
                                proto->hasSelfOrAssociatedTypeRequirements(),
-                               S.addTypeRef(proto->getSuperclass()),
+                               S.addDeclRef(proto->getSuperclassDecl()),
                                rawAccessLevel,
                                dependencyTypeIDs);