pgsql: Clean up a few failures to set collation fields in expression no

Clean up a few failures to set collation fields in expression nodes.

I'm not sure these have any non-cosmetic implications, but I'm not sure
they don't, either. In particular, ensure the CaseTestExpr generated
by transformAssignmentIndirection to represent the base target column
carries the correct collation, because parse_collate.c won't fix that.
Tweak lsyscache.c API so that we can get the appropriate collation
without an extra syscache lookup.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/b23c9fa9293c54a3829093d207be37a7b42cb630

Modified Files
--------------
src/backend/optimizer/path/pathkeys.c | 3 +-
src/backend/optimizer/plan/createplan.c | 2 +
src/backend/optimizer/util/clauses.c | 2 +-
src/backend/optimizer/util/predtest.c | 4 +++
src/backend/parser/parse_coerce.c | 2 +
src/backend/parser/parse_target.c | 31 ++++++++++++++++++++++---
src/backend/utils/adt/ruleutils.c | 6 +++-
src/backend/utils/cache/lsyscache.c | 38 +++++--------------------------
src/include/utils/lsyscache.h | 5 +--
9 files changed, 49 insertions(+), 44 deletions(-)