glr2.cc: make the example more C++

Currently the example really looks like C.  Instead of a union of
structs to implement the AST, use a hierarchy.  It would be nice to
feature a C++17 version with std variants.

* examples/c++/glr/c++-types.yy (Node, free_node, new_nterm)
(new_term): Move into...
* examples/c++/glr/ast.hh: here, a proper C++ hierarchy.

examples/c++/glr/local.mk

@@ -37,6 +37,7 @@ $(%C%_c___types_OBJECTS): $(cxx_types_sources_generated)
 if ENABLE_CXX
   check_PROGRAMS += %D%/c++-types
   nodist_%C%_c___types_SOURCES =		\
+    %D%/ast.hh                                  \
     %D%/c++-types.cc				\
     %D%/c++-types.hh
   # Don't use gnulib's system headers.