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

src/backend/commands/extension.c

@@ -703,6 +703,9 @@ execute_sql_string(const char *sql, const char *filename)
 		List	   *stmt_list;
 		ListCell   *lc2;
 
+		/* Be sure parser can see any DDL done so far */
+		CommandCounterIncrement();
+
 		stmt_list = pg_analyze_and_rewrite(parsetree,
 										   sql,
 										   NULL,