The jsii kernel needs to attach metadata to instances that are sent
across the process boundary. While these properties use symbol names to
eliminate the risk for collisions with user-defined properties, they
were inadvertently set as `enumerable`, causing those to show in outputs
of `JSON.stringify` and to be copied over as part of spat expressions
(e.g: `{...obj}`), which resulted in the new value sharing the same
identity as the previous one when sent over the wire.

Caused: aws/aws-cdk#17876

Co-authored-by: Nick Lynch <nlynch@amazon.com>