[SE-0470] Prohibit isolated conformances in dynamic casts marked as such

Certain dynamic casts cannot work safely with isolated conformances,
regardless of what executor the code runs on. For such cases, reject
all attempts to conform to the type.

stdlib/public/runtime/Private.h

@@ -721,7 +721,8 @@ public:
       const OpaqueValue *value,
       const Metadata *type,
       ProtocolDescriptorRef protocol,
-      const WitnessTable **conformance);
+      const WitnessTable **conformance,
+      bool prohibitIsolatedConformances);
 
   /// Construct type metadata for the given protocol.
   const Metadata *