pgsql: Fix coerce_to_target_type for coerce_type's klugy handling of CO

Fix coerce_to_target_type for coerce_type's klugy handling of COLLATE.

Because coerce_type recurses into the argument of a CollateExpr,
coerce_to_target_type's longstanding code for detecting whether coerce_type
had actually done anything (to wit, returned a different node than it
passed in) was broken in 9.1. This resulted in unexpected failures in
hide_coercion_node; which was not the latter's fault, since it's critical
that we never call it on anything that wasn't inserted by coerce_type.
(Else we might decide to "hide" a user-written function call.)

Fix by removing and replacing the CollateExpr in coerce_to_target_type
itself. This is all pretty ugly but I don't immediately see a way to make
it nicer.

Per report from Jean-Yves F. Barbier.

Branch
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REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/188f1b928205bf33ce29887eeeee26ce9227908f

Modified Files
--------------
src/backend/parser/parse_coerce.c | 28 +++++++++++++++++++++++++++-
src/test/regress/expected/collate.out | 3 +++
src/test/regress/sql/collate.sql | 5 +++++
3 files changed, 35 insertions(+), 1 deletions(-)