randgen: do not create computed columns with bad volatilities

[mgartner]

Some cast to STRING types has been given an incorrect volatility. For
example, REGCLASS->STRING casts are immutable when they should be stable
(see #74286 and #74553 for more details).

Creating computed column expressions with such a cast can cause logical
correctness bugs and internal errors. The volatilities cannot be fixed
without causing backward incompatibility. This commit prevents randgen
from creating computed columns with these casts so that sqlsmith and TLP
do not repetitively find these known volatility bugs.

Informs #74727

Release note: None

[cockroach-teamcity]

This change is 

[rafiss]

thanks!

Reviewable status: complete! 0 of 0 LGTMs obtained

[mgartner]

TFTR!

bors r+

[craig]

Build succeeded:

• GitHub CI (Cockroach)

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error creating merge commit from 8b5f53c to blathers/backport-release-21.1-75161: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 21.1.x failed. See errors above.

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error creating merge commit from 8b5f53c to blathers/backport-release-21.2-75161: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 21.2.x failed. See errors above.

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