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.

src/muscle-tab.c

@@ -447,6 +447,7 @@ muscle_percent_variable_update (char const *variable,
     { "api.push_pull",              "api.push-pull",             muscle_keyword },
     { "api.tokens.prefix",          "api.token.prefix",          muscle_code },
     { "extends",                    "api.parser.extends",        muscle_keyword },
+    { "filename_type",              "api.filename.type",         muscle_code },
     { "final",                      "api.parser.final",          muscle_keyword },
     { "implements",                 "api.parser.implements",     muscle_keyword },
     { "lex_symbol",                 "api.token.constructor",     -1 },