[Serialization] Serialize the isUserAccessible bit on functions

rdar://problem/53891642
SR-7460

lib/Serialization/Serialization.cpp

@@ -3437,6 +3437,7 @@ public:
                            rawAccessLevel,
                            fn->needsNewVTableEntry(),
                            S.addDeclRef(fn->getOpaqueResultTypeDecl()),
+                           fn->isUserAccessible(),
                            nameComponentsAndDependencies);
 
     writeGenericParams(fn->getGenericParams());