c++: introduce api.value.automove

Based on work by Frank Heckenbach.
See http://lists.gnu.org/archive/html/bug-bison/2018-04/msg00000.html
and http://lists.gnu.org/archive/html/bug-bison/2018-09/msg00019.html.

* data/lalr1.cc (b4_rhs_value): Use YY_MOVE api.rhs.automove is set.
* doc/bison.texi (%define Summary): Document api.rhs.automove.
* examples/variant-11.yy: Use it.

* tests/local.at (AT_AUTOMOVE_IF): New.
* tests/c++.at (Variants): Check move semantics.

examples/variant-11.yy

@@ -20,6 +20,7 @@
 %defines
 %define api.token.constructor
 %define api.value.type variant
+%define api.value.automove
 %define parse.assert
 %locations
 
@@ -96,11 +97,11 @@ result:
 
 list:
   %empty     { /* Generates an empty string list */ }
-| list item  { $$ = std::move ($1); $$.emplace_back (std::move ($2)); }
+| list item  { $$ = $1; $$.emplace_back ($2); }
 ;
 
 item:
-  TEXT    { $$ = std::move ($1); }
+  TEXT    { $$ = $1; }
 | NUMBER  { $$ = make_string_uptr (to_string ($1)); }
 ;
 %%