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