c++: symbols can be empty, so use it

c++: symbols can be empty, so use it

The previous patches ensure that symbols (symbol_type and
stack_symbol_type) can be empty, cleared, and their emptiness can be
checked.  Therefore, yyempty, which codes whether yyla is empty or
not, is now useless.

In C skeletons (e.g., yacc.c), the fact that the lookahead is empty is
coded by "yychar = YYEMPTY", which is exactly what this patch
restores, since yychar/yytoken corresponds to yyla.type.

* data/lalr1.cc (yyempty): Remove.
Rather, depend on yyla.empty ().