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.

tests/calc.at

@@ -303,7 +303,7 @@ line:
 ;
 
 exp:
-  NUM                { $$ = $1;             }
+  NUM
 | exp '=' exp
   {
     if ($1 != $3)