generate the default semantic action

Currently, in C, the default semantic action is implemented by being
always run before running the actual user semantic action.  As a
consequence, when the user action is run, $$ is already set as $1.

In C++ with variants, we don't do that, since we cannot manipulate the
semantic value without knowing its exact type.  When variants are
enabled, the only guarantee is that $$ is default contructed and ready
to the used.

Some users still would like the default action to be run with
variants.  Frank Heckenbach's parser in
C++17 (http://lists.gnu.org/archive/html/bug-bison/2018-04/msg00011.html)
provides this feature, but relying on std::variant's dynamic typing,
which we forbid in lalr1.cc.

The simplest seems to be actually generating the default semantic
action (in all languages/skeletons).  This makes the pre-action (that
sets $$ to $1) useless.  But...  maybe some users depend on this, in
spite of the comments that clearly warn againt this.  So let's not
turn this off just yet.

* src/reader.c (grammar_rule_check_and_complete): Rename as...
(grammar_rule_check_and_complete): this.
Install the default semantic action when applicable.
* examples/variant-11.yy, examples/variant.yy, tests/calc.at:
Exercise the default semantic action, even with variants.

NEWS

@@ -68,6 +68,27 @@ GNU Bison NEWS
 
   The new examples/variant-11.yy shows these features in action.
 
+*** C++: The implicit default semantic action is always run
+
+  When variants are enabled, the default action was not run, so
+
+    exp: "number"
+
+  was equivalent to
+
+    exp: "number"  {}
+
+  It now behaves like in all the other cases, as
+
+    exp: "number"  { $$ = $1; }
+
+  possibly using std::move if automove is enabled.
+
+  We do not expect backward compatibility issues.  However, beware of
+  forward compatibility issues: if you rely on default actions with
+  variants, be sure to '%require "3.2"' to avoid older versions of Bison to
+  generate incorrect parsers.
+
 *** C++: Renaming location.hh
 
   When both %defines and %locations are enabled, Bison generates a