PR libstdc++/91012 fixfilesystem_error::what() string

PR libstdc++/91012 fixfilesystem_error::what() string

When I refactored the filesystem_error code I changed it to only use the
constructor parameter in the what() string, instead of the string
returned by system_error::what(). That meant it no longer included the
description of the error_code that system_error adds. This restores the
previous behaivour, as encouraged by the standard ("Implementations
should include the system_error::what() string and the pathnames of
path1 and path2 in the native format in the returned string").

	PR libstdc++/91012
	* src/c++17/fs_path.cc (filesystem_error::_Impl): Use a string_view
	for the what_arg parameters.
	(filesystem_error::filesystem_error): Pass system_error::what() to
	the _Impl constructor.
	* testsuite/27_io/filesystem/filesystem_error/cons.cc: Ensure that
	filesystem_error::what() contains system_error::what().

From-SVN: r272739