* include/bits/shared_ptr_base.h (__shared_ptr::owner_before)
	(__weak_ptr::owner_before, _Sp_owner_less::operator()): Add noexcept
	specifiers as per LWG 2873 and LWG 2942.
	* testsuite/20_util/owner_less/noexcept.cc: New.
	* testsuite/20_util/shared_ptr/observers/owner_before.cc: Test
	noexcept guarantees.
	* testsuite/20_util/weak_ptr/observers/owner_before.cc: Likewise.

From-SVN: r248874