RENAME TABLE a TO b, c TO d grouped the target of the first clause and
the source of the second into a single IdentifierList, because the comma
between the two clauses was treated as an identifier-list separator.
Detect the rename layout by the TO keywords flanking both sides of the
comma and leave such a comma ungrouped, so each renamed table stays a
separate Identifier. Genuine target lists such as GRANT ... TO a, b are
left untouched.
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