pgsql: Fix old bug with coercing the result of a COLLATE expression.

Fix old bug with coercing the result of a COLLATE expression.

There are hacks in parse_coerce.c to push down a requested coercion
to below any CollateExpr that may appear.  However, we did that even
if the requested data type is non-collatable, leading to an invalid
expression tree in which CollateExpr is applied to a non-collatable
type.  The fix is just to drop the CollateExpr altogether, reasoning
that it's useless.

This bug is ten years old, dating to the original addition of
COLLATE support.  The lack of field complaints suggests that there
aren't a lot of user-visible consequences.  We noticed the problem
because it would trigger an assertion in DefineVirtualRelation if
the invalid structure appears as an output column of a view; however,
in a non-assert build, you don't see a crash just a (subtly incorrect)
complaint about applying collation to a non-collatable type.  I found
that by putting the incorrect structure further down in a view, I could
make a view definition that would fail dump/reload, per the added
regression test case.  But CollateExpr doesn't do anything at run-time,
so this likely doesn't lead to any really exciting consequences.

Per report from Yulin Pei.  Back-patch to all supported branches.

Discussion: https://postgr.es/m/HK0PR01MB22744393C474D503E16C8509F4709@HK0PR01MB2274.apcprd01.prod.exchangelabs.com

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/a6158a4d9c2e6b8af0d4217b8d80afeff8adec0e

Modified Files
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src/backend/parser/parse_coerce.c     | 29 ++++++++++++++++++-----------
src/test/regress/expected/collate.out | 16 +++++++++++++++-
src/test/regress/sql/collate.sql      |  6 ++++++
3 files changed, 39 insertions(+), 12 deletions(-)