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.

tests/input.at

@@ -2220,6 +2220,7 @@ AT_DATA([[input.y]],
 %define namespace "foo"
 %define variant
 %define parser_class_name {parser}
+%define filename_type {filename}
 %%
 start: %empty;
 ]])
@@ -2244,6 +2245,10 @@ input.y:5.1-34: warning: deprecated directive: '%define parser_class_name {parse
     5 | %define parser_class_name {parser}
       | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
       | %define api.parser.class {parser}
+input.y:6.1-32: warning: deprecated directive: '%define filename_type {filename}', use '%define api.filename.type {filename}' [-Wdeprecated]
+    6 | %define filename_type {filename}
+      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+      | %define api.filename.type {filename}
 input.y:2.1-40: error: invalid value for %define Boolean variable 'lr.keep-unreachable-state'
     2 | %define lr.keep_unreachable_states maybe
       | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~