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: /messages/by-id/1423433.1652722406@sss.pgh.pa.us

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/36ccca3dbac3663b5487cd5966b345009b182629

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(+)