api.value.type: implement proper support, check, and document

* data/c.m4 (b4_symbol_type_register, b4_type_define_tag)
(b4_symbol_value_union, b4_value_type_setup_union)
(b4_value_type_setup_variant, b4_value_type_setup):
New.
(b4_value_type_define): Use it to set up properly the type.
Handle the various possible values of api.value.type.
* data/c++.m4 (b4_value_type_declare): Likewise.
* data/lalr1.cc (b4_value_type_setup_variant): Redefine.

* tests/types.at: New.
Exercise all the C/C++ skeletons with different types of
api.value.type values.
* tests/local.mk, tests/testsuite.at: Use it.

* doc/bison.texi (%define Summary): Document api.value.type.
* NEWS: Advertise it, together with api.token.constructor.

data/c++.m4

@@ -118,15 +118,16 @@ m4_define([b4_token_enums],
 # ---------------------
 # Declare semantic_type.
 m4_define([b4_value_type_declare],
+[b4_value_type_setup[]dnl
 [    /// Symbol semantic values.
-m4_ifdef([b4_union_members],
-[    union semantic_type
+]m4_bmatch(b4_percent_define_get([api.value.type]),
+[^%union\|union$],
+[[    union semantic_type
     {
-b4_user_union_members
-    };],
-[m4_if(b4_tag_seen_flag, 0,
-[[    typedef int semantic_type;]],
-[[    typedef ]b4_api_PREFIX[STYPE semantic_type;]])])])
+]b4_user_union_members[
+    };]],
+[^$], [],
+[[    typedef ]b4_percent_define_get([api.value.type])[ semantic_type;]])])
 
 
 # b4_public_types_declare