When fixing Item_func_plus in ORDER BY clause field c is searched in all
opened tables, but because c is an alias it wasn't found there.

This patch adds a flag to select_lex which allows Item_field::fix_fields() 
to look up in select's item_list to find aliased fields.


sql/item.cc:
  Fix bug#7672 Unknown column error in order clause
  When fixing fields in ORDER BY clause allow Item_field::fix_fields() to look up items in select's item list to find aliased fields.
sql/sql_lex.cc:
   Fix bug#7672 Unknown column error in order clause
sql/sql_lex.h:
  Fix bug#7672 Unknown column error in order clause
  Added flag to select_lex allowing Item_field::fix_fields to look up items in select's item list.
sql/sql_select.cc:
  Fix bug#7672 Unknown column error in order clause
mysql-test/t/select.test:
  Test case for bug#7672 Unknown column error in order clause
mysql-test/r/select.result:
  Test case for bug#7672 Unknown column error in order clause