c++: provide backward compatibility on by_type

To write unit tests for their scanners, some users depended on
symbol_type::token():

    Lexer lex("12345");
    symbol_type t = lex.nextToken();
    assert(t.token() == token::INTLIT);
    assert(t.value.as<int>() == 12345);

But symbol_type::token() was removed in Bison 3.5 because it relied on
a conversion table.  So users had to find other patterns, such as

    assert(t.type_get() == by_type(token::INTLIT).type_get());

which relies on several private implementation details.

As part of transitioning from "token type" to "token kind", and making
this a public and documented interface, "by_type" was renamed
"by_kind" and "type_get()" was renamed as "kind()".  The latter had
backward compatibility mechanisms, not the former.

In Bison 3.6 none of this should be used, but rather

    assert(t.kind() == symbol_kind::S_INTLIT);

Reported by Pramod Kumbhar.
https://lists.gnu.org/r/bug-bison/2020-05/msg00012.html

* data/skeletons/c++.m4 (by_type): Make it an alias to by_kind.

THANKS

@@ -155,6 +155,7 @@ Petr Machata              pmachata@redhat.com
 Pho                       pho@cielonegro.org
 Piotr Gackiewicz          gacek@intertel.com.pl
 Piotr Marcińczyk          piomar123@gmail.com
+Pramod Kumbhar            pramod.s.kumbhar@gmail.com
 Quentin Hocquet           hocquet@gostai.com
 Quoc Peyrot               chojin@lrde.epita.fr
 R Blake                   blakers@mac.com

data/skeletons/c++.m4

@@ -414,6 +414,9 @@ m4_define([b4_symbol_type_define],
       symbol_kind_type kind_;
     };
 
+    /// Backward compatibility for a private implementation detail.
+    typedef by_kind by_type;
+
     /// "External" symbols: returned by the scanner.
     struct symbol_type : basic_symbol<by_kind>
     {]b4_variant_if([[