[v9.2] DROP Reworks Part.0 - 'missing_ok' support of get_object_address

The attached patch is a preparation to rework implementation of DROP statement
using a common code. That intends to apply get_object_address() to resolve name
of objects to be removed, and eventually minimizes the number of places to put
permission checks.

Its first step is an enhancement of get_object_address; to accept 'missing_ok'
argument to handle cases when IF EXISTS clause is supplied.
If 'missing_ok' was true and the supplied name was not found, the patched
get_object_address() returns an ObjectAddress with InvalidOid as objectId.
If 'missing_ok' was false, its behavior is not changed.

Thanks,
--
KaiGai Kohei <kaigai(at)kaigai(dot)gr(dot)jp>