sql: improved partial index implication with virtual columns

The issue tracks figuring out how to prove implication (and thus enable the use of the partial index) for cases like:

```
CREATE TABLE t (
  k INT PRIMARY KEY,
  a INT,
  b INT,
  c INT AS (b + 10) VIRTUAL,
  INDEX (a) WHERE c IN (10, 20, 30)
)

SELECT k FROM t WHERE c = 20
```

Jira issue: CRDB-2993