libstdc++: Add test for chrono::leap_seconds ostream insertion

libstdc++: Add test for chrono::leap_seconds ostream insertion

Also add a comment to the three-way comparison oeprator for
chrono::leap_seconds, noting the deviation from the spec (which is
functionally equivalent). What we implement is the originally proposed
resolution to LWG 3383, which should compile slightly more efficiently
than the final accepted resolution.

libstdc++-v3/ChangeLog:

	* include/std/chrono (leap_seconds): Add comment.
	* testsuite/std/time/leap_seconds/io.cc: New test.