pgsql: Ensure commands in extension scripts see the results of precedin

Ensure commands in extension scripts see the results of preceding DDL.

Due to a missing CommandCounterIncrement() call, parsing of a non-utility
command in an extension script would not see the effects of the immediately
preceding DDL command, unless that command's execution ends with
CommandCounterIncrement() internally ... which some do but many don't.
Report by Philippe Beaudoin, diagnosis by Julien Rouhaud.

Rather remarkably, this bug has evaded detection since extensions were
invented, so back-patch to all supported branches.

Discussion: https://postgr.es/m/2cf7941e-4e41-7714-3de8-37b1a8f74dff@free.fr

Branch
------
REL9_6_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/d56b8b41b32b3f5c576d6fc2b2c9af3ae4b55511

Modified Files
--------------
src/backend/commands/extension.c | 3 +++
src/test/modules/test_extensions/test_ext3--1.0.sql | 6 ++++++
2 files changed, 9 insertions(+)