pgsql: SQL/JSON: Some fixes to JsonBehavior expression casting

SQL/JSON: Some fixes to JsonBehavior expression casting

1. Remove the special case handling when casting the JsonBehavior
expressions to types with typmod, like 86d33987 did for the casting
of SQL/JSON constructor functions.

2. Fix casting for fixed-length character and bit string types by
using assignment-level casts. This is again similar to what
86d33987 did, but for ON ERROR / EMPTY expressions.

3. Use runtime coercion for the boolean ON ERROR constants so that
using fixed-length character string types, for example, for an
EXISTS column doesn't cause a "value too long for type
character(n)" when the parser tries to coerce the default ON ERROR
value "false" to that type, that is, even when clause is not
specified.

4. Simplify the conditions of when to use runtime coercion vs
creating the cast expression in the parser itself. jsonb-valued
expressions are now always coerced at runtime and boolean
expressions too if the target type is a string type for the
reasons mentioned above.

Tests are taken from a patch that Jian He posted.

Reported-by: Jian He <jian(dot)universality(at)gmail(dot)com>
Author: Jian He <jian(dot)universality(at)gmail(dot)com>
Author: Amit Langote <amitlangote09(at)gmail(dot)com>
Discussion: https://postgr.es/m/CACJufxEo4sUjKCYtda0_qt9tazqqKPmF1cqhW9KBOUeJFqQd2g@mail.gmail.com
Backpatch-through: 17

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/74c96699be3f53c058c8794c07952221b9625b4f

Modified Files
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src/backend/parser/parse_expr.c | 100 ++++++++++++++++-------
src/test/regress/expected/sqljson_jsontable.out | 29 +++++--
src/test/regress/expected/sqljson_queryfuncs.out | 66 +++++++++++----
src/test/regress/sql/sqljson_jsontable.sql | 5 +-
src/test/regress/sql/sqljson_queryfuncs.sql | 16 +++-
5 files changed, 164 insertions(+), 52 deletions(-)