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: https://postgr.es/m/2ddcc188-e37c-a0be-32bf-a56b07c3559e@proxel.se
Backpatch-through: 9.5

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/3a2923a9bfd8e769d5bafeef4284fbf1ba953cbf

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(+)