support duplicate entries in the opaque_type_storage

[lcnr]

Necessary for the new solver as we may unify keys when eagerly resolving for canonical queries. See the relevant comment when instantiating query responses:

            // We eagerly resolve inference variables when computing the query response.
            // This can cause previously distinct opaque type keys to now be structurally equal.
            //
            // To handle this, we store any duplicate entries in a separate list to check them
            // at the end of typeck/borrowck. We could alternatively eagerly equate the hidden
            // types here. However, doing so is difficult as it may result in nested goals and
            // any errors may make it harder to track the control flow for diagnostics.
            if let Some(prev) = prev {
                self.delegate.add_duplicate_opaque_type(key, prev, self.origin_span);
            }

This will be far more relevant with #140497.

r? @compiler-errors