MDEV-35616: Add basic optimizer support for virtual column

(Review input addressed)

After this patch, the optimizer can handle virtual column expressions
in WHERE/ON clauses. If the table has an indexed virtual column:

  ALTER TABLE t1
    ADD COLUMN vcol INT AS (col1+1),
    ADD INDEX idx1(vcol);

and the query uses the exact virtual column expression:

  SELECT * FROM t1 WHERE col1+1 <= 100

then the optimizer will be able use index idx1 for it.

This is achieved by walking the WHERE/ON clauses and replacing instances
of virtual column expression (like "col1+1" above) with virtual column's
Item_field (like "vcol"). The latter can be processed by the optimizer.

Replacement is considered (and done) only in items that are potentially
usable to the range optimizer.

sql/item.h

@@ -2463,6 +2463,9 @@ public:
   bool cache_const_expr_analyzer(uchar **arg);
   Item* cache_const_expr_transformer(THD *thd, uchar *arg);
 
+  bool vcol_subst_analyzer(uchar **);
+  virtual Item* vcol_subst_transformer(THD *thd, uchar *arg) { return this; }
+
   virtual Item* propagate_equal_fields(THD*, const Context &, COND_EQUAL *)
   {
     return this;