Fix two bugs related to the casts optimizations.

The protocol_lookup the testcase from Interpreter test-cases exposed two bugs, once I tried to compiler with with -O:
 - SILCloner was generating an open_existential_ref from an open_existential_metatype instruction during cloning even if the existential in question was not a class existential.
 - DynamicCasts was not considering the fact that subclasses of a given class may implement a protocol, even if the class does not implement it.

Swift SVN r27260

lib/SIL/DynamicCasts.cpp

@@ -130,6 +130,13 @@ classifyDynamicCastToProtocol(CanType source,
   if (auto *CD = source.getClassOrBoundGenericClass()) {
     if (canClassOrSuperclassesHaveExtensions(CD, isWholeModuleOpts))
       return DynamicCastFeasibility::MaySucceed;
+    // Derived types may conform to the protocol.
+    if (!CD->isFinal()) {
+      // TODO: If it is a private type or internal type and we
+      // can prove that there are no derived types conforming to a
+      // protocol, then we can still return WillFail.
+      return DynamicCastFeasibility::MaySucceed;
+    }
   }
 
   // If the source type is private or target protocol is private,