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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REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/06cdab9df413238c24f839f8806164c36b75f0e5

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(+)