libstdc++: Refactor filesystem::path encoding conversions

libstdc++: Refactor filesystem::path encoding conversions

Adjust the __detail::__effective_range overloads so they always return a
string or string view using std::char_traits, because we don't care
about the traits of an incoming string.

Use std::contiguous_iterator in the __effective_range(const Source&)
overload, to allow returning a basic_string_view in more cases. For the
non-contiguous casecl in both __effective_range and __string_from_range,
return a std::string instead of std::u8string when the value type of the
range is char8_t.  These changes avoid unnecessary basic_string
temporaries.

Also simplify __string_from_range(Iter, Iter) to not need
std::__to_address for the contiguous case.

Combine the _S_convert(string_type) and _S_convert(const T&) overloads
into a single _S_convert(T) function which also avoids the dangling
view problem of PR 102592 (should that recur somehow).

libstdc++-v3/ChangeLog:

	* include/bits/fs_path.h (__detail::__is_contiguous): New
	variable template to identify contiguous iterators.
	(__detail::__unified_char8_t): New alias template to decide when
	to treat char8_t as char without encoding conversion.
	(__detail::__effective_range(const basic_string<C,T>&)): Use
	std::char_traits<C> for returned string view.
	(__detail::__effective_range(const basic_string_view<C,T>&)):
	Likewise.
	(__detail::__effective_range(const Source&)): Use
	__is_contiguous to detect mode cases of contiguous iterators.
	Use __unified_char8_t to return a std::string instead of
	std::u8string.