Overload std::distance and std::advance for path::iterator

Overload std::distance and std::advance for path::iterator

Although filesystem::path::iterator is only a bidirectional iterator,
the underlying sequence has random access iterators (specifically, raw
pointers). This means std::distance and std::advance can be implemented
more efficiently than the generic versions which apply ++ and --
repeatedly.

	PR libstdc++/71044 (partial)
	* include/bits/fs_path.h (__path_iter_distance, __path_iter_advance):
	New friend functions to implement std::distance and std::advance more
	efficiently.
	(distance, advance): Add overloads for path::iterator.
	* testsuite/27_io/filesystem/path/itr/components.cc: Test new
	overload.

From-SVN: r267057