[patch] Detect non-member operator&() overloads in std::experimental::optional

[Jonathan Wakely]

On 12/11/14 00:16 +0000, Jonathan Wakely wrote:
>The _Has_addressof trait only detected an overloaded operator&() as a
>member function, this also checks for a non-member.

Also clean up the tests by removing some unused headers.

Tested x86_64-linux, committed to trunk.
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