sql_yacc.yy: add %type create_field for field_spec and column_def

We'll introduce a new token type "create_field" in sql_yacc.yy:

%union {

...

  Create_field *create_field;

...

}

and change data type of the rules "field_spec" from and "column_def from NONE to create_field.

The rule "field_spec" will return the value created by this code:

field_spec:

          field_ident

          {

            LEX *lex=Lex;

            Create_field *f= new Create_field();

         ...

The rule "column_def" will further return the value created by its "field_spec" part.