c++: by default, use const std::string for file names

Reported by Martin Blais and Yuriy Solodkyy.
https://lists.gnu.org/r/help-bison/2020-05/msg00011.html
https://lists.gnu.org/r/bug-bison/2020-06/msg00038.html

While at it, modernize filename_type as api.filename.type and document
it properly.

* data/skeletons/c++.m4 (filename_type): Rename as...
(api.filename.type): this.
Default to const std::string.
* data/skeletons/location.cc (position, location): Expose the
filename_type type.
Use api.filename.type.
* doc/bison.texi (%define Summary): Document api.filename.type.
(C++ Location Values): Document position::filename_type.
* src/muscle-tab.c (muscle_percent_variable_update): Ensure backward
compatibility.
* tests/c++.at: Check that using const file names is ok.
tests/input.at: Check backward compat.

THANKS

@@ -112,6 +112,7 @@ Marc Autret               autret_m@epita.fr
 Marc Mendiola             mmendiol@usc.edu
 Marc Schönefeld           marc.schoenefeld@gmx.org
 Mark Boyall               wolfeinstein@gmail.com
+Martin Blais              blais@furius.ca
 Martin Jacobs             martin.jacobs@arcor.de
 Martin Mokrejs            mmokrejs@natur.cuni.cz
 Martin Nylin              martin.nylin@linuxmail.org
@@ -215,6 +216,7 @@ Wolfram Wagner            ww@mpi-sb.mpg.de
 Wwp                       subscript@free.fr
 xolodho                   xolodho@gmail.com
 Yuichiro Kaneko           spiketeika@gmail.com
+Yuriy Solodkyy            solodon@gmail.com
 Zack Weinberg             zack@codesourcery.com
 江 祖铭                    jjzuming@outlook.com
 長田偉伸                   cbh34680@iret.co.jp