[patch] Make std::forward_list safe against overloaded operator&

[Jonathan Wakely]

Similar to yesterday's patch for std::list.

Also removes some invalid downcasts of the _Node_base sentinel's
address to _Node*. The casts aren't needed anyway, as we only access
members of the base class through those pointers.

Also removes some redundant upcasts using *static_cast<Base*>(&x)
which will fail if there's an operator& overload associated with Base.
We can just rely on the implicit conversion to Base& instead.

Tested powerpc64le-linux, committed to trunk.
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