sql: disallow set-returning functions in CASE expressions

[mgartner]

CRDB allows set-returning functions in CASE expressions, while Postgres does not. For example, this succeeds in CRDB but fails in Postgres:

SELECT CASE WHEN true THEN generate_series(1, 3) ELSE 1 END;

The behavior of such an expression is not well-defined, and the results are unexpected, if not down right incorrect. For example:

defaultdb> SELECT CASE WHEN false THEN generate_series(1, 3) ELSE 1 END;
  case
--------
     1
     1
     1
(3 rows)

I'd expect this expression to return only a single row.

We should probably disallow set-returning functions within CASE expressions (and other conditional expressions).

Jira issue: CRDB-24525