The fix currently implemented for the LIKE function only.
Later it can be extended for other functions.

Added new field to the FIELD class:

Key_map part_of_prefixkey;

Added new method to the FIELD class:

/**
  Get covering prefix keys.

  @retval covering prefix keys.
*/
Key_map get_covering_prefix_keys();

Added new field to the Item_field class:

/*
   TRUE if it's possible to set prefix key as covering
   depending on internal logic of the function.
*/
bool can_use_prefix_key;

Added new methods to the Item_field class:

/*
   The method sets 'can_use_prefix_key' to TRUE before
   calling ::fix_fields() for the arguments. This method
   is called only for the argument which can be used for
   prefix key reading. If 'can_use_prefix_key' is TRUE
   we don't remove prefix keys from the covering keys.
   It allows to make decision if the key is covering later
   in appropriate function(currently LIKE function only).
*/
void set_can_use_prefix_key()

Changes in Item_func_like class:

In Item_func_like::fix_fields first argument is set as 'can_use_prefix_key'
before arguments fixing. After function arguments are fixed new method
Item_func_like::check_covering_prefix_keys() is called. It sets covering keys
depending on constant part of the wild patttern.

Added new method:
Item_func_like::check_covering_prefix_keys();

Changes in table.cc:

In TABLE::mark_column_used, intersect TABLE::covering_keys with the union
of part_of_key and part_of_prefixkey.

Added new method:
/**
  Update covering keys depending on max read key length.

  @param[in]   field             Pointer to field object
  @param[in]   key_read_length   Max read key length
*/
void TABLE::update_covering_prefix_keys(Field *field, uint16 key_read_length);