[v3 patch] Remove __gnu_cxx::__alloc_traits::_S_nothrow_swap()

[Jonathan Wakely]

As I suggested recently, this removes the _S_nothrow_swap() function
because it's redundant: allocators are not allowed to throw exceptions
when swapped, so we can assume they don't.

So now exception specifications for swapping containers don't depend
on allocators.

This also adds a _GLIBCXX_NOEXCEPT_IF macro for helping to define
conditional noexcept guarantees.

Tested powerpc64le-linux, committed to trunk.

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