Users get confused when testing a JSON literal true/false in boolean
context like
a) NOT(JSON value containing literal)
b) WHERE JSON value containing literal
c) JSON value containing literal IS TRUE/FALSE.
And it's a bit of our fault, as this used to work in 5.7.
But it stopped working in 8.0 after
WL#12358: Ensure that all predicates in SQL conditions are complete
and
WL#4245 Convert NOT IN and NOT EXISTS in WHERE/ON to an antijoin
(both introduced in 8.0.17).
We cannot easily restore the behaviour of 5.7, and it's questionable
if we should. The SQL standard doesn't ask that JSON values be
implicitly cast to SQL values when needed, it rather expects the user
to use JSON_VALUE for that.

To help users a bit we start emitting a warning if they have
constructs like above, and a suggestion (using JSON_VALUE).

Note that we already did this for e.g. BETWEEN:
+--------------------------------------+
| cast(false as json) between -1 and 3 |
+--------------------------------------+
|                                    1 |
+--------------------------------------+
Warning (Code 1235): This version of MySQL doesn't yet support 'comparison of JSON in the BETWEEN operator'

We do not do the same for IS NULL (which has problems too, see Bug #
25822217), because there are cases where JSON functions do return SQL
NULL, so we cannot warn each time a JSON function is an argument of IS
NULL, it may be a valid test. Even though some JSON functions don't
return SQL NULL when operating on a JSON literal null (and, that's
when some users think they should... see bug 85755).

Reviewed-by: Catalin Besleaga <catalin.besleaga@oracle.com>
Change-Id: I4ce5d06b4d6c40a4ce68220375a4505d349df235