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_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/96dbbf828c5f188e9fec0b706ad33c20900f918d

Modified Files
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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(+)