MDEV-6679 Different optimizer plan for "a BETWEEN 'string' AND ?" and "a BETWEEN ? AND 'string'"

Item_string::eq() and Item_param::eq() in string context behaved differently.
Introducing a new class Item_basic_value to share the eq() code between
literals (Item_int, Item_double, Item_string, Item_null) and Item_param.

sql/sql_string.h

@@ -540,6 +540,15 @@ public:
     }
     return TRUE;
   }
+  bool bin_eq(const String *other) const
+  {
+    return length() == other->length() &&
+           !memcmp(ptr(), other->ptr(), length());
+  }
+  bool eq(const String *other, CHARSET_INFO *cs) const
+  {
+    return !sortcmp(this, other, cs);
+  }
 };