Add PART_INDIRECT_KEY_FLAG

This is to mark that a field is indirectly part of a key, which simplifes
checking if we need to have this field up to date to evaluate a key.

For example:
CREATE TABLE t1 (a int, b int as (a) virtual,
                 c int as (b) virtual, index(c))
would mark a and b with PART_INDIRECT_KEY_FLAG.
c is marked with PART_KEY_FLAG as before.

include/mysql_com.h

@@ -179,6 +179,7 @@ enum enum_indicator_type
 #define BINCMP_FLAG	131072U		/* Intern: Used by sql_yacc */
 #define GET_FIXED_FIELDS_FLAG (1U << 18) /* Used to get fields in item tree */
 #define FIELD_IN_PART_FUNC_FLAG (1U << 19)/* Field part of partition func */
+#define PART_INDIRECT_KEY_FLAG (1U << 20)
 
 /**
   Intern: Field in TABLE object for new version of altered table,