make immutability tests in to_json and to_jsonb complete

make immutability tests in to_json and to_jsonb complete

Complete the TODOs in to_json_is_immutable() and to_jsonb_is_immutable()
by recursing into container types (arrays, composites, ranges, multiranges,
domains) to check element/sub-type mutability, rather than conservatively
returning "mutable" for all arrays and composites.

The shared logic is factored into a single json_check_mutability() function
in jsonfuncs.c, with the existing exported functions as thin wrappers.
Composite type inspection uses lookup_rowtype_tupdesc() (typcache) instead
of relation_open() to avoid unnecessary lock acquisition in the optimizer.

Range and multirange types are now also checked recursively: if the
subtype's conversion is immutable, the range is considered immutable
for JSON purposes, even though range_out is generically marked STABLE.
This is a behavioral change: range types with immutable subtypes (e.g.,
int4range) can now appear in expression indexes via JSON_ARRAY/JSON_OBJECT,
whereas previously they were conservatively rejected.

Add regression tests for JSON_ARRAY and JSON_OBJECT mutability with
expression indexes and generated columns, covering arrays, composites,
domains, ranges, multiranges and combinations thereof.

Author: Jian He <jian.universality@gmail.com>
Reviewed-by: Andrew Dunstan <andrew@dunslane.net>
Discussion: https://postgr.es/m/CACJufxFz=OsXQdsMJ-cqoqspD9aJrwntsQP-U2A-UaV_M+-S9g@mail.gmail.com
Commitfest: https://commitfest.postgresql.org/patch/5759