Allow affixing SQLCommenter comment if comment already exists in statement

[jschaf]

There's certain comments that are necessary to include in a SQL statement. The use case I have in mind in Postgres hints 1.

  /*+
    HashJoin(a b)
    SeqScan(a)
  */
 EXPLAIN SELECT *
    FROM pgbench_branches b
    JOIN pgbench_accounts a ON b.bid = a.bid
   ORDER BY a.aid;

The spec states:

If a comment already exists within a SQL statement, we MUST NOT mutate that statement.

That means we can never use SQLCommenter with pg hint comments.

Proposal: modify spec to optionally allow affixing a comment. Alternately, narrow the spec to "if a comment already exists as the last token in a sql statement". When parsing choose the last comment if there's more than one.