libstdc++: Add missing parts of LWG 3480 for directory iterators [PR117560]

libstdc++: Add missing parts of LWG 3480 for directory iterators [PR117560]

It looks like I only read half the resolution of LWG 3480 and decided we
already supported it. As well as making the non-member overloads of end
take their parameters by value, we need some specializations of the
enable_borrowed_range and enable_view variable templates.

libstdc++-v3/ChangeLog:

	PR libstdc++/117560
	* include/bits/fs_dir.h (enable_borrowed_range, enable_view):
	Define specializations for directory iterators, as per LWG 3480.
	* testsuite/27_io/filesystem/iterators/lwg3480.cc: New test.