pgsql: Fix PL/pgSQL's handling of anycompatiblemultirange.

Fix PL/pgSQL's handling of anycompatiblemultirange.

When validating a PL/pgSQL function with polymorphic arguments or
result type, we substitute a plausible concrete type for each
polymorphic type. anycompatiblemultirange was missed in both places
that do this, when multiranges were added. As a result, CREATE
FUNCTION with an anycompatiblemultirange argument failed with
"PL/pgSQL functions cannot accept type anycompatiblemultirange" when
check_function_bodies is on.

Also, a function returning anycompatiblemultirange was compiled by
the validator with an int4 result type, and since the function cache
key doesn't include the result type, a later call in the same session
with int4range arguments reused that entry and failed trying to cast
the multirange result to integer.

Map anycompatiblemultirange to int4multirange, as is done for
anymultirange, to fix both issues.

Author: Jeevan Chalke <jeevan(dot)chalke(at)enterprisedb(dot)com>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/CAM2+6=VEXCo7tPaxX+9_zN8SJ9DiPp2NTj_sPGN+Tb0wWTBp=w@mail.gmail.com
Backpatch-through: 14

Branch
------
REL_18_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/a34d61616134c75694e99abf8beb58b7b82b3455

Modified Files
--------------
src/backend/utils/cache/funccache.c | 1 +
src/pl/plpgsql/src/pl_comp.c | 3 ++-
2 files changed, 3 insertions(+), 1 deletion(-)