This adds a check to weak_ptr::_M_assign() so that calling
__enable_shared_from_this_helper twice with the same pointer won't
change which shared_ptr object the weak_ptr shares ownership with.

On the lib reflector Peter Dimov convinced me that the
boost::enable_shared_from_this behaviour is preferable to what we do
now. I'm writing a proposal to specify this in the standard, but am
changing it now in our implementation.

Tested powerpc64le-linux, committing to trunk.