pgsql: Add CREATE COLLATION IF NOT EXISTS clause

Add CREATE COLLATION IF NOT EXISTS clause

The core of the functionality was already implemented when
pg_import_system_collations was added. This just exposes it as an
option in the SQL command.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/6d16ecc646d21b39092970c591fd0f73b4cfc26b

Modified Files
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doc/src/sgml/ref/create_collation.sgml | 15 +++++++++++++--
src/backend/commands/collationcmds.c | 4 ++--
src/backend/nodes/copyfuncs.c | 1 +
src/backend/nodes/equalfuncs.c | 1 +
src/backend/parser/gram.y | 20 ++++++++++++++++++++
src/backend/tcop/utility.c | 3 ++-
src/include/commands/collationcmds.h | 2 +-
src/include/nodes/parsenodes.h | 1 +
src/test/regress/expected/collate.linux.utf8.out | 4 ++++
src/test/regress/sql/collate.linux.utf8.sql | 2 ++
10 files changed, 47 insertions(+), 6 deletions(-)