sql: do not collect statistics on virtual columns

[mgartner]

PR #68312 intended to update the behavior of CREATE STATISTICS to
prevent statistics collection on virtual computed columns. However, it
failed to account for multi-column statistics and for
CREATE STATISTICS statements that explicitly reference virtual
columns. This commit accounts for these two cases.

This prevents internal errors from occuring when the system tries to
collect statistics on NOT NULL virtual columns. Virtual column values
are not included in the primary index. So when the statistics
job reads the primary index to sample the virtual column, it assumes the
value is null, which violates the column's NOT NULL constraint. This
violation causes an error.

Fixes #71080

Release note (bug fix): A bug has been fixed which caused internal
errors when collecting statistics on tables with virtual computed
columns.

[cockroach-teamcity]

This change is 

[mgartner]

This should be backported to 21.1 along with #68312.

[rytaft]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @mgartner)

[cucaroach]

Reviewable status: complete! 2 of 0 LGTMs obtained (waiting on @rytaft)

[mgartner]

TFTRs!

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 d9eed2b to blathers/backport-release-21.1-71105: 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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