pgsql: Fix support for CREATE TABLE IF NOT EXISTS AS EXECUTE

Fix support for CREATE TABLE IF NOT EXISTS AS EXECUTE

The grammar IF NOT EXISTS for CTAS is supported since 9.5 and documented
as such, however the case of using EXECUTE as query has never been
covered as EXECUTE CTAS statements and normal CTAS statements are parsed
separately.

Author: Andreas Karlsson
Discussion: /messages/by-id/2ddcc188-e37c-a0be-32bf-a56b07c3559e@proxel.se
Backpatch-through: 9.5

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/331a613e9d363febfee8508e8de545fd84624758

Modified Files
--------------
src/backend/parser/gram.y | 18 ++++++++++++++++++
src/test/regress/expected/create_table.out | 14 ++++++++++++++
src/test/regress/sql/create_table.sql | 8 ++++++++
3 files changed, 40 insertions(+)