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.

examples/variant-11.yy

@@ -101,7 +101,7 @@ list:
 ;
 
 item:
-  TEXT    { $$ = $1; }
+  TEXT
 | NUMBER  { $$ = make_string_uptr (to_string ($1)); }
 ;
 %%

examples/variant.yy

@@ -89,7 +89,7 @@ list:
 ;
 
 item:
-  TEXT    { std::swap ($$, $1); }
+  TEXT
 | NUMBER  { $$ = to_string ($1); }
 ;
 %%