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
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/75aba11ec5edf7e2f0f45a8bc67f4f20d0a33282

Modified Files
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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(+)