By only defining these operators as friends (with no namespace-scope
declaration) they can only be found by ADL and do not participate in
overload resolution for arguments of types other than path.

	LWG 2989 hide path iostream operators from normal lookup
	* include/bits/fs_path.h (operator<<, operator>>): Define inline as
	friends.
	* testsuite/27_io/filesystem/path/io/dr2989.cc: New.

From-SVN: r261711