Fix collations when we call transformWhereClause from outside the parser.

Previous patches took care of assorted places that call transformExpr from
outside the main parser, but I overlooked the fact that some places use
transformWhereClause as a shortcut for transformExpr + coerce_to_boolean.
In particular this broke collation-sensitive index WHERE clauses, as per
report from Thom Brown.  Trigger WHEN and rule WHERE clauses too.

I'm not forcing initdb for this fix, but any affected indexes, triggers,
or rules will need to be dropped and recreated.