[Runtime] Authenticate relative witness tables in _checkWitnessTableIsolation.

When SWIFT_STDLIB_USE_RELATIVE_PROTOCOL_WITNESS_TABLES is set, witness table pointers are signed. We need to authenticate them before using them.

rdar://148687148

Author: mikeash

stdlib/public/runtime/ProtocolConformance.cpp

@@ -444,6 +444,12 @@ static bool _checkWitnessTableIsolation(
   llvm::ArrayRef<const void *> conditionalArgs,
   ConformanceExecutionContext &context
 ) {
+#if SWIFT_STDLIB_USE_RELATIVE_PROTOCOL_WITNESS_TABLES && SWIFT_PTRAUTH
+  wtable = ptrauth_auth_data(wtable,
+        ptrauth_key_process_independent_data,
+        SpecialPointerAuthDiscriminators::RelativeProtocolWitnessTable);
+#endif
+
   // If there's no protocol conformance descriptor, do nothing.
   auto description = wtable->getDescription();
   if (!description)