Refactor collating-sequence handling as a fix for ticket [71e333e7d2e642].

Refactor collating-sequence handling as a fix for ticket [71e333e7d2e642].
The Expr.pColl field is removed from the Expr object.  The COLLATE operator
now becomes a separate instance of Expr in the expression tree.  The code
generator looks up the correct collating function as needed, rather than
referring to Expr.pColl.

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