schemachanger: `Replace` discards element initial state, breaking rollback

`builderState.replaceElement`, overrides an existing element state with `initial=ABSENT` and discards the element's true prior state. This is a problem for rollback because it relies on the elements initial status to revert the mutations (`scpb.CurrentState.Rollback`). Therefore, a Replace'd element is driven to `ABSENT` rather than restored to its pre-change value.

This is currently masked: every Replace caller (`CREATE OR REPLACE FUNCTION`, `TRUNCATE` function/trigger reference rewrites, owner changes) lands only in non-revertible phases, so the wrong rollback path is never taken. It becomes a live correctness bug once transactional DDL lets a Replace share a job with a revertible post-commit phase (e.g. a backfill): a failed job "reverts" but leaves the Replace'd payload in its mid-transaction state.

We should fix this in the element/rollback model.

Jira issue: CRDB-67833

Epic CRDB-65516