Make filesystem::path comparison operators hidden friends (LWG 3065)

Make filesystem::path comparison operators hidden friends (LWG 3065)

This change revealed two testsuite bugs where some string comparisons
only compiled by converting the strings to filesystem::path objects.

	* include/bits/fs_path.h (operator<, operator<=, operator>)
	(operator>=, operator==, operator!=): Make hidden friends, as per
	LWG 3065.
	* testsuite/27_io/filesystem/path/native/string-char8_t.cc: Fix
	string type in test.
	* testsuite/27_io/filesystem/path/native/string.cc: Likewise.

From-SVN: r270558