libstdc++: Define function to throw filesystem_error [PR 98471]

libstdc++: Define function to throw filesystem_error [PR 98471]

Fix ordering problem on Windows targets where filesystem_error was used
before being defined.

libstdc++-v3/ChangeLog:

	PR libstdc++/98471
	* include/bits/fs_path.h (__throw_conversion_error): New
	function to throw or abort on character conversion errors.
	(__wstr_from_utf8): Move definition after filesystem_error has
	been defined. Use __throw_conversion_error.
	(path::_S_convert<_EcharT>): Use __throw_conversion_error.
	(path::_S_str_convert<_CharT, _Traits, _Allocator>): Likewise.
	(path::u8string): Likewise.