Use hidden friends for directory iterator comparisons

Use hidden friends for directory iterator comparisons

The equality operators for directory iterators are not explicitly
specified in the standard, they're only required to meet the iterator
requirements. This means we don't need to declare them at namespace
scope and can implement them as hidden friends.

Also add 'noexcept' to directory_iterator's dereference operators.

	* include/bits/fs_dir.h (directory_iterator::operator*)
	(directory_iterator::operator->): Add noexcept.
	(operator==, operator!=): Replace namespace-scope equality operators
	for directory iterators with hidden friends.

From-SVN: r270175