libstdc++: Fix filesystem::u8path for mingw targets (PR 95392)

When I refactored filesystem::path string conversions in
r11-587-584d52b0 I failed to update the
mingw-specific code in filesystem::u8path, causing a bootstrap failure.

This fixes it, and further refactors the mingw-specific code along the
same lines as the previous commit. All conversions from UTF-8 strings to
wide strings now use the same helper function, __wstr_from_utf8.

	PR libstdc++/95392
	* include/bits/fs_path.h (path::_S_to_string): Move to
	namespace-scope and rename to ...
	(__detail::__string_from_range): ... this.
	[WINDOWS] (__detail::__wstr_from_utf8): New function template to
	convert a char sequence containing UTF-8 to wstring.
	(path::_S_convert(Iter, Iter)): Adjust call to _S_to_string.
	(path::_S_convert_loc(Iter, Iter, const locale&)): Likewise.
	(u8path(InputIterator, InputIterator)) [WINDOWS]: Use
	__string_from_range to obtain a contiguous range and
	__wstr_from_utf8 to obtain a wide string.
	(u8path(const Source&)) [WINDOWS]: Use __effective_range to
	obtain a contiguous range and __wstr_from_utf8 to obtain a wide
	string.
	(path::_S_convert(const _EcharT*, const _EcharT)) [WINDOWS]:
	Use __wstr_from_utf8.