pgsql: Fix bugs in polymorphic-argument resolution for multiranges.

Fix bugs in polymorphic-argument resolution for multiranges.

We failed to deal with an UNKNOWN-type input for
anycompatiblemultirange; that should throw an error indicating
that we don't know how to resolve the multirange type.

We also failed to infer the type of an anycompatiblerange output
from an anycompatiblemultirange input or vice versa.

Per bug #17066 from Alexander Lakhin. Back-patch to v14
where multiranges were added.

Discussion: https://postgr.es/m/17066-16a37f6223a8470b@postgresql.org

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/b7ea0e8c41f1e512923267a57cd08df63115b783

Modified Files
--------------
src/backend/parser/parse_coerce.c | 316 ++++++++++++++++-------------
src/test/regress/expected/polymorphism.out | 68 ++++++-
src/test/regress/expected/rangefuncs.out | 2 +-
src/test/regress/sql/polymorphism.sql | 48 +++++
4 files changed, 293 insertions(+), 141 deletions(-)