pgsql: Fix multiple assignments to a column of a domain type.

Fix multiple assignments to a column of a domain type.

We allow INSERT and UPDATE commands to assign to the same column more than
once, as long as the assignments are to subfields or elements rather than
the whole column. However, this failed when the target column was a domain
over array rather than plain array. Fix by teaching process_matched_tle()
to look through CoerceToDomain nodes, and add relevant test cases.

Also add a group of test cases exercising domains over array of composite.
It's doubtless accidental that CREATE DOMAIN allows this case while not
allowing straight domain over composite; but it does, so we'd better make
sure we don't break it. (I could not find any documentation mentioning
either side of that, so no doc changes.)

It's been like this for a long time, so back-patch to all supported
branches.

Discussion: https://postgr.es/m/4206.1499798337@sss.pgh.pa.us

Branch
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REL9_2_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/55204850a9de72ab0a7bc7b90131d46fad7a89cd

Modified Files
--------------
src/backend/rewrite/rewriteHandler.c | 31 ++++++++++
src/test/regress/expected/domain.out | 111 +++++++++++++++++++++++++++++++++--
src/test/regress/sql/domain.sql | 48 +++++++++++++++
3 files changed, 186 insertions(+), 4 deletions(-)