libstdc++: Fix memory leak in std::barrier destructor [PR123378]

libstdc++: Fix memory leak in std::barrier destructor [PR123378]

When I replaced the std::unique_ptr member in r16-997-gef632273a90657 I
should have added an explicit delete[] operation to replace the effects
of the unique_ptr destructor.

libstdc++-v3/ChangeLog:

	PR libstdc++/123378
	* include/std/barrier (~__tree_barrier_base): Define.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>