[Patch, libstdc++/68877] Reimplement __is_[nothrow_]swappable

[Daniel KrÃgler <daniel dot kruegler at gmail dot com>]

This is a reimplementation of __is_swappable and
__is_nothrow_swappable according to

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4511.html

and also adds a missing usage of __is_nothrow_swappable in the swap
overload for arrays. Strictly speaking the latter change differs from
the Standard specification which requires the expression
noexcept(swap(*a, *b)) to be used. On the other hand the Standard is
broken in this regard, as pointed out by

http://cplusplus.github.io/LWG/lwg-active.html#2554

Thanks,

- Daniel

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