[mlir][StorageUniquer] Properly call the destructor on non-trivially destructible storage instances

This allows for storage instances to store data that isn't uniqued in the context, or contain otherwise non-trivial logic, in the rare situations that they occur. Storage instances with trivial destructors will still have their destructor skipped. A consequence of this is that the storage instance definition must be visible from the place that registers the type.

Differential Revision: https://reviews.llvm.org/D98311