Since commit 72dd233d we were trying to obtain object addressing
information in sql_drop event triggers, but that caused failures when
the drops involved user mappings.  This addition enables that to work
again.  Naturally, pg_get_object_address can work with these objects
now, too.

I toyed with the idea of removing DropUserMappingStmt as a node and
using DropStmt instead in the DropUserMappingStmt grammar production,
but that didn't go very well: for one thing the messages thrown by the
specific code are specialized (you get "server not found" if you specify
the wrong server, instead of a generic "user mapping for ... not found"
which you'd get it we were to merge this with RemoveObjects --- unless
we added even more special cases).  For another thing, it would require
to pass RoleSpec nodes through the objname/objargs representation used
by RemoveObjects, which works in isolation, but gets messy when
pg_get_object_address is involved.  So I dropped this part for now.

Reviewed by Stephen Frost.