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.

THANKS

@@ -12,6 +12,7 @@ Andreas Schwab            schwab@suse.de
 Andrew Suffield           asuffield@users.sourceforge.net
 Angelo Borsotti           angelo.borsotti@gmail.com
 Anthony Heading           ajrh@ajrh.net
+Antonio Silva Correia     amsilvacorreia@hotmail.com
 Arnold Robbins            arnold@skeeve.com
 Art Haas                  ahaas@neosoft.com
 Baron Schwartz            baron@sequent.org
@@ -85,6 +86,7 @@ Matt Rosing               rosing@peakfive.com
 Michael Felt              mamfelt@gmail.com
 Michael Hayes             m.hayes@elec.canterbury.ac.nz
 Michael Raskin            7c6f434c@mail.ru
+Michel d'Hooge            michel.dhooge@gmail.com
 Michiel De Wilde          mdewilde.agilent@gmail.com
 Mickael Labau             labau_m@epita.fr
 Mike Castle               dalgoda@ix.netcom.com