libstdc++/3150

[Jason Merrill]

I just noticed that back in November Benjamin removed the destructors from
all the libsupc++ classes, pursuant to DR 266.  I think this is wrong; the
DR only means that the destructors should be removed from the descriptions
in the standard.  Those classes will have destructors whether they are
written explicitly or not, and writing them explicitly allows the vtable
heuristic to work.

Jason