MDEV-18195 ASAN use-after-poison in my_strcasecmp_utf8 / Item::eq upon prepared statement with ORDER BY NAME_CONST

ASAN noticed a freed memory access during EXECUTE in this script:
  PREPARE stmt FROM "SELECT 'x' ORDER BY NAME_CONST( 'f', 'foo' )";
  EXECUTE stmt;

In case of a PREPARE statement, all Items, including Item_name_const,
are created on Prepared_statement::main_mem_root.
Item_name_const::fix_fields() did not take this into account
and could allocate the value of Item::name on a wrong memory root,
in this code:

  if (is_autogenerated_name)
  {
    set_name(thd, item_name->c_ptr(), (uint) item_name->length(),
             system_charset_info);
  }

When fix_fields() is called in the reported SQL script, THD's arena already
points to THD::main_mem_root rather than to Prepared_statement::main_mem_root,
so Item::name was allocated on THD::main_mem_root.
Then, at the end of the dispatch_command() for the PREPARE statement,
THD::main_mem_root got cleared. So during EXECUTE, Item::name
pointed to an already freed memory.

This patch changes the code to set the implicit name for Item_name_const
at the constructor time rather than at fix_fields time. This guarantees
that Item_name_const and its Item::name always reside on the same memory root.

Note, this change makes the code for Item_name_const symmetric with other
constant-alike items that set their default implicit names at the constructor
call time rather than at fix_fields() time:
- Item_string
- Item_int
- Item_real
- Item_decimal
- Item_null
- Item_param