Grammar object type refactoring
Unify the grammar of COMMENT, DROP, and SECURITY LABEL further. They
all effectively just take an object address for later processing, so
we can make the grammar more generalized. Some extra checking about
which object types are supported can be done later in the statement
execution.
Discussion: https://www.postgresql.org/message-id/flat/
163c00a5-f634-ca52-fc7c-
0e53deda8735%402ndquadrant.com