pgsql: Add select_common_typmod()

Add select_common_typmod()

This accompanies select_common_type() and select_common_collation().
Typmods were previously combined using hand-coded logic in several
places.  The logic in select_common_typmod() isn't very exciting, but
it makes the code more compact and readable in a few locations, and in
the future we can perhaps do more complicated things if desired.

As a small enhancement, the type unification of the direct and
aggregate arguments of hypothetical-set aggregates now unifies the
typmod as well using this new function, instead of just dropping it.

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://www.postgresql.org/message-id/flat/97df3af9-8b5e-fb7f-a029-3eb7e80d7af9@2ndquadrant.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f893e68d761adbee7f888109b1adf76151e3e17a

Modified Files
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src/backend/parser/analyze.c      | 26 ++++++--------------------
src/backend/parser/parse_clause.c | 23 ++++++-----------------
src/backend/parser/parse_coerce.c | 37 +++++++++++++++++++++++++++++++++++++
src/backend/parser/parse_func.c   |  8 ++++++--
src/include/parser/parse_coerce.h |  2 ++
5 files changed, 57 insertions(+), 39 deletions(-)