When adding a functional index on an expression that returned a
character string, the index was sometimes used for predicates that
compared strings using another collation than the indexed expression.
This could cause wrong results.

substitute_gc_expression() has a check that prevents use of functional
indexes which don't have the same collation as the corresponding
expression in the query. WL#8955 unintentionally made the check
looser, by only checking the collation if both the hidden generated
column backing the functional index and the indexed expression had the
type VARCHAR. Since JSON_UNQUOTE returns LONGTEXT, the collation was
not checked for functional indexes on JSON_UNQUOTE.

Fixed by making the check stricter and checking the collation for all
string types.

Change-Id: I4cc5b0fdff505a4a9c8bee75ae8d1c93c9ed09b4