Add back the missing unique_ptr swap

[BillyONeal]

Add back the missing unique_ptr swap damaged by (Microsoft-internal, pre-GitHub commit) f84d346b33eff67c3e5038fbb32342a82abdb4f2

Resolves DevCom-754487 / VSO-1000729

Description

It looks like when I moved around allocator support for std::unique_ptr to fix our many allocator::construct bugs, I accidentally deleted this overload. Everything appeared to continue to work because the default std::swap does the right thing, although less efficiently and instantiating more templates than necessary.

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