C++: support variadic emplace

Suggested by Askar Safin.
http://lists.gnu.org/archive/html/bug-bison/2018-12/msg00006.html

* data/variant.hh: Implement.
* tests/types.at: Check.
* doc/bison.texi: Document.

doc/bison.texi

@@ -11088,20 +11088,17 @@ in midrule actions.  It is mandatory to use typed midrule actions
 (@pxref{Typed Midrule Actions}).
 
 @deftypemethod {semantic_type} {T&} emplace<T> ()
-Initialize, but leave empty.  Return a reference to where the actual value
-may be stored.  Requires that the variant was not initialized yet.
+@deftypemethodx {semantic_type} {T&} emplace<T> (const T& @var{t})
+Available in C++98/C++03 only.  Default construct/copy-construct from
+@var{t}.  Return a reference to where the actual value may be stored.
+Requires that the variant was not initialized yet.
 @end deftypemethod
 
-@deftypemethod {semantic_type} {T&} emplace<T> (const T& @var{t})
-Initialize, and copy-construct from @var{t}.  Available in C++98/C++03 only.
+@deftypemethod {semantic_type} {T&} emplace<T, U> (U&&... @var{u})
+Available in C++11 and later only.  Build a variant of type @code{T} from
+the variadic forwarding references @var{u...}.
 @end deftypemethod
 
-@deftypemethod {semantic_type} {T&} emplace<T, U> (U&& @var{u})
-Build a variant of type @code{T} from the forwarding reference @var{u}.
-Available in C++11 and later only.
-@end deftypemethod
-
-
 @strong{Warning}: We do not use Boost.Variant, for two reasons.  First, it
 appeared unacceptable to require Boost on the user's machine (i.e., the
 machine on which the generated parser will be compiled, not the machine on