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            [email protected]
 Andrew Suffield           [email protected]
 Angelo Borsotti           [email protected]
 Anthony Heading           [email protected]
+Antonio Silva Correia     [email protected]
 Arnold Robbins            [email protected]
 Art Haas                  [email protected]
 Baron Schwartz            [email protected]
@@ -85,6 +86,7 @@ Matt Rosing               [email protected]
 Michael Felt              [email protected]
 Michael Hayes             [email protected]
 Michael Raskin            [email protected]
+Michel d'Hooge            [email protected]
 Michiel De Wilde          [email protected]
 Mickael Labau             [email protected]
 Mike Castle               [email protected]