My initial implementation of experimental::any was too dumb, it
assumed you could just swap the bytes of _Storage, and return it from
functions like any POD type, but if the _Storage object contains a
non-trivially-copyable object internally (not on the heap) then it's
wrong to treat it as a POD.

This patch makes the _Storage object non-copyable and fixes all the
uses of it.

Tested powerpc64le-linux, committed to trunk.