Introduce make_symbol.

Introduce make_symbol.

make_symbol provides a means to construct a full symbol (kind, value,
location) in a single shot.  It is meant to be a Symbol constructor,
parameterized by the symbol kind so that overloading would prevent
incorrect kind/value pairs.  Unfortunately parameterized constructors do
not work well in C++ (unless the parameter also appears as an argument,
which is not acceptable), hence the use of a function instead of a
constructor.

	* data/lalr1.cc (b4_symbol_constructor_declaration_)
	(b4_symbol_constructor_declarations)
	(b4_symbol_constructor_specialization_)
	(b4_symbol_constructor_specializations)
	(b4_symbol_constructor_definition_)
	(b4_symbol_constructor_definitions): New.
	Use them where appropriate to generate declaration, declaration of
	the specializations, and implementations of the templated
	overloaded function "make_symbol".
	(variant::variant): Always define a default ctor.
	Also provide a copy ctor.
	(symbol_base_type, symbol_type): New ctor overloads for value-less
	symbols.
	(symbol_type): Now public, so that functions such as yylex can use
	it.