pgsql: Fix omissions in support for the "regcollation" type.

Fix omissions in support for the "regcollation" type.

The patch that added regcollation doesn't seem to have been too
thorough about supporting it everywhere that other reg* types
are supported.  Fix that.  (The find_expr_references omission
is moderately serious, since it could result in missing expression
dependencies.  The others are less exciting.)

Noted while fixing bug #17483.  Back-patch to v13 where
regcollation was added.

Discussion: https://postgr.es/m/1423433.1652722406@sss.pgh.pa.us

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/bb30410b9f4cae1ec74f120833b74ecc0b1de7d2

Modified Files
--------------
src/backend/catalog/dependency.c   | 7 +++++++
src/backend/utils/adt/selfuncs.c   | 2 ++
src/backend/utils/cache/catcache.c | 1 +
3 files changed, 10 insertions(+)