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