c++: exhibit a safe symbol_type

Instead of introducing make_symbol (whose name, btw, somewhat
infringes on the user's "name space", if she defines a token named
"symbol"), let's make the construction of symbol_type safer, using
assertions.

For instance with:

    %token ':' <std::string> ID <int> INT;

generate:

    symbol_type (int token, const std::string&);
    symbol_type (int token, const int&);
    symbol_type (int token);

It does mean that now named token constructors (make_ID, make_INT,
etc.) go through a useless assert, but I think we can ignore this: I
assume any decent compiler will inline the symbol_type ctor inside the
make_TOKEN functions, which will show that the assert is trivially
verified, hence I expect no code will be emitted for it.  And anyway,
that's an assert, NDEBUG controls it.

* data/c++.m4 (symbol_type): Turn into a subclass of
basic_symbol<by_type>.
Declare symbol constructors when variants are enabled.
* data/variant.hh (_b4_type_constructor_declare)
(_b4_type_constructor_define): Replace with...
(_b4_symbol_constructor_declare, _b4_symbol_constructor_def): these.
Generate symbol_type constructors.
* doc/bison.texi (Complete Symbols): Document.
* tests/types.at: Check.

data/c++.m4

@@ -332,7 +332,17 @@ m4_define([b4_symbol_type_declare],
     };
 
     /// "External" symbols: returned by the scanner.
-    typedef basic_symbol<by_type> symbol_type;
+    struct symbol_type : basic_symbol<by_type>
+    {]b4_variant_if([[
+      /// Superclass.
+      typedef basic_symbol<by_type> super_type;
+
+      /// Empty symbol.
+      symbol_type () {};
+
+      /// Constructor for valueless symbols, and symbols from each type.
+]b4_type_foreach([_b4_symbol_constructor_declare])[
+    ]])[};
 ]])