[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

@@ -444,8 +444,18 @@ static bool _checkWitnessTableIsolation(
   llvm::ArrayRef<const void *> conditionalArgs,
   ConformanceExecutionContext &context
 ) {
-  // If there's no protocol conformance descriptor, do nothing.
+#if SWIFT_STDLIB_USE_RELATIVE_PROTOCOL_WITNESS_TABLES && SWIFT_PTRAUTH
+  wtable = ptrauth_auth_data(
+      wtable, ptrauth_key_process_independent_data,
+      SpecialPointerAuthDiscriminators::RelativeProtocolWitnessTable);
+  auto description = lookThroughOptionalConditionalWitnessTable(
+                         reinterpret_cast<const RelativeWitnessTable *>(wtable))
+                         ->getDescription();
+#else
   auto description = wtable->getDescription();
+#endif
+
+  // If there's no protocol conformance descriptor, do nothing.
   if (!description)
     return false;