libstdc++: Fix filesystem::path comparisons for C++23

libstdc++: Fix filesystem::path comparisons for C++23

In C++23 there is a basic_string_view(Range&&) constructor, which is
constrained to take a range (specifically, a contiguous_range). When the
filesystem::path comparison operators call lhs.compare(rhs) the overload
taking a string_view is considered, which means checking whether path
satisfies the range concept. That satisfaction result changes depending
whether path::iterator is complete, which is ill-formed; no diagnostic
required. To avoid the problem, this change ensures that the overload
resolution is performed in a context where path::iterator is complete
and the range concept is satisfied. (The result of overload resolution
is always that the compare(const path&) overload is the best match, but
we still have to consider the compare(basic_string_view<value_type>) one
to decide if it even participates in overload resolution).

For std::filesystem::path we can't define the comparison operators later
in the file, because they are hidden friends, so a new helper is
introduced that gets defined when everything else is complete.

For std::experimental::filesystem::path we can just move the definitions
of the comparison operators later in the file.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	* include/bits/fs_path.h (operator==, operator<=>): Use new
	_S_compare function.
	(path::_S_compare): New function to call path::compare in a
	context where path::iterator is complete.
	* include/experimental/bits/fs_path.h (operator<, operator==):
	Define after path::iterator is complete.
	* testsuite/27_io/filesystem/path/native/conv_c++23.cc: New
	test.
	* testsuite/experimental/filesystem/path/native/conv_c++23.cc:
	New test.