[Serialization] Allow conformances with no witness if allowing errors

Note that deserialization already handles this case when recovery is
enabled.

lib/Serialization/Serialization.cpp

@@ -1460,7 +1460,8 @@ void Serializer::writeASTBlockEntity(
       data.push_back(addDeclRef(req));
       data.push_back(addDeclRef(witness.getDecl()));
       assert(witness.getDecl() || req->getAttrs().hasAttribute<OptionalAttr>()
-             || req->getAttrs().isUnavailable(req->getASTContext()));
+             || req->getAttrs().isUnavailable(req->getASTContext())
+             || allowCompilerErrors());
 
       // If there is no witness, we're done.
       if (!witness.getDecl()) return;