[Runtime] Handle relative witness tables in _checkWitnessTableIsolation.

We need to use lookThroughOptionalConditionalWitnessTable when relative witness tables are being used.

rdar://148687148
(cherry picked from commit f01d46c)

Author: mikeash

stdlib/public/runtime/ProtocolConformance.cpp

@@ -444,8 +444,15 @@ 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
+  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;