pgsql: Fix maintenance hazards caused by ill-considered use of default:

Fix maintenance hazards caused by ill-considered use of default: cases.

Remove default cases from assorted switches over ObjectClass and some
related enum types, so that we'll get compiler warnings when someone
adds a new enum value without accounting for it in all these places.

In passing, re-order some switch cases as needed to match the declaration
of enum ObjectClass. OK, that's just neatnik-ism, but I dislike code
that looks like it was assembled with the help of a dartboard.

Discussion: https://postgr.es/m/20170512221010.nglatgt5azzdxjlj@alvherre.pgsql

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e84c0195980f24b1c7f857b88834c1dcaf20a102

Modified Files
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src/backend/catalog/dependency.c | 21 ++-
src/backend/catalog/objectaddress.c | 305 ++++++++++++++++++-----------------
src/backend/commands/alter.c | 38 ++++-
src/backend/commands/event_trigger.c | 75 ++++++---
4 files changed, 259 insertions(+), 180 deletions(-)