c++: provide a means to clear symbols

c++: provide a means to clear symbols

The symbol destructor is currently the only means to clear a symbol.
Unfortunately during error recovery we might have to clear the
lookahead, which is a local variable (yyla) that has not yet reached
its end of scope.

Rather that duplicating the code to destroy a symbol, or rather than
destroying and recreating yyla, let's provide a means to clear a
symbol.

Reported by Antonio Silva Correia, with an analysis from Michel d'Hooge.
<http://savannah.gnu.org/support/?108481>

* data/c++.m4, data/lalr1.cc (basis_symbol::clear, by_state::clear)
(by_type::clear): New.
(basic_symbol::~basic_symbol): Use clear.