pgsql: Fix collation handling in plpgsql functions.

Fix collation handling in plpgsql functions.

Make plpgsql treat the input collation as a polymorphism variable, so
that we cache separate plans for each input collation that's used in a
particular session, as per recent discussion. Propagate the input
collation to all collatable input parameters.

I chose to also propagate the input collation to all declared variables of
collatable types, which is a bit more debatable but seems to be necessary
for non-astonishing behavior. (Copying a parameter into a separate local
variable shouldn't result in a change of behavior, for example.) There is
enough infrastructure here to support declaring a collation for each local
variable to override that default, but I thought we should wait to see what
the field demand is before adding such a feature.

In passing, remove exec_get_rec_fieldtype(), which wasn't used anywhere.

Documentation patch to follow.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/a4425e3200f2c2578bdf94bb6234169115746c9f

Modified Files
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src/pl/plpgsql/src/gram.y | 21 ++++--
src/pl/plpgsql/src/pl_comp.c | 98 ++++++++++++++++------
src/pl/plpgsql/src/pl_exec.c | 79 ++++++++++++-----
src/pl/plpgsql/src/plpgsql.h | 18 +++-
src/test/regress/expected/collate.linux.utf8.out | 82 ++++++++++---------
src/test/regress/sql/collate.linux.utf8.sql | 11 ++-
6 files changed, 207 insertions(+), 102 deletions(-)