release-21.2: sql: stale descriptor state could be observed in crdb_internal tables

[fqazi]

Backport 1/1 commits from #76381.

fixes #70008
Release justification: low risk and this issue can lead to incorrect results for customers querying information via crdb_internal tables.

/cc @cockroachdb/release

---

Fixes: #75825

Previously, when an uncommitted descriptor was added
to a descriptor collection, we would not reset the cache of
all immutable descriptors (used by some crdb_internal functions).
This was inadequate because this could result in
incorrect results from crdb_internal tables which returned
information related to constraints, columns. To address this,
this patch will reset the descriptor cache for all
fetched descriptors when an uncommitted descriptor is
added into the collection.

Release note (bug fix): Certain crdb_internal tables could return incorrect
information due to cached table descriptor information.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues or test-only changes.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is 

[ajwerner]

I think this deserves a release note. Otherwise, LGTM

[fqazi]

@rafiss TFTR!