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.

New tests are from a patch that Jian He posted. Outputs of some
existing tests change because the coercion now happens at runtime
instead of at parse time.

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
------
REL_17_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/847ee701bd3a6d222ab8f67923e308ee1a70a2f8

Modified Files
--------------
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(-)