pgsql: Fix Assert failure in XMLTABLE parser

Fix Assert failure in XMLTABLE parser

In an XMLTABLE expression, columns can be marked NOT NULL, and the
parser internally fabricates an option named "is_not_null" to
represent this. However, the parser also allows users to specify
arbitrary option names. This creates a conflict: a user can
explicitly use "is_not_null" as an option name and assign it a
non-Boolean value, which violates internal assumptions and triggers an
assertion failure.

To fix, this patch checks whether a user-supplied name collides with
the internally reserved option name and raises an error if so.
Additionally, the internal name is renamed to "__pg__is_not_null" to
further reduce the risk of collision with user-defined names.

Reported-by: Евгений Горбанев <gorbanyoves(at)basealt(dot)ru>
Author: Richard Guo <guofenglinux(at)gmail(dot)com>
Reviewed-by: Alvaro Herrera <alvherre(at)kurilemu(dot)de>
Discussion: https://postgr.es/m/6bac9886-65bf-4cec-96bd-e304159f28db@basealt.ru
Backpatch-through: 15

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d3716d4b135756e38775a29a5ca26e17bd0efafc

Modified Files
--------------
src/backend/parser/gram.y | 15 +++++++++++----
src/test/regress/expected/xml.out | 4 ++++
src/test/regress/expected/xml_1.out | 4 ++++
src/test/regress/expected/xml_2.out | 4 ++++
src/test/regress/sql/xml.sql | 2 ++
5 files changed, 25 insertions(+), 4 deletions(-)