scbuild: CatalogReader's MustReadDescriptor method should return stored descriptors

Presently, this method returns uncommitted descriptors. In multi-statement txns, this can lead to inconsistencies because they're fed to scdecomp which infers the element statuses on a best-effort basis. As a consequence, the inferred statuses might actually differ from those in the incumbent target state.

This doesn't matter much yet because explicit transactions are not supported in production but we should do the right early on: decomp the descriptor such as it was before the txn and upsert with the descriptor's elements which are in the incumbent target state. Uncommitted descriptors should never be used as a source of truth in scbuild.

Jira issue: CRDB-29689

Start at scbuild's CatalogReader.MustReadDescriptor method and trace how its result is passed to scdecomp. The change is done when scbuild uses stored descriptors from before the transaction and upserts elements from the incumbent target state rather than using uncommitted descriptors as its source of truth.

Written by the indexing model from the issue text.