pgsql: Support user mappings in get_object_address

Support user mappings in get_object_address

Since commit 72dd233d3ef 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.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/890192e99af5db1d15d5bb73f3f1044faa1d2758

Modified Files
--------------
src/backend/catalog/objectaddress.c          |   78 +++++++++++++++++++++++++-
src/backend/commands/event_trigger.c         |    1 +
src/include/nodes/parsenodes.h               |    1 +
src/test/regress/expected/event_trigger.out  |    9 ++-
src/test/regress/expected/object_address.out |   19 +++++--
src/test/regress/sql/event_trigger.sql       |    5 +-
src/test/regress/sql/object_address.sql      |    9 +--
7 files changed, 109 insertions(+), 13 deletions(-)