folly: fix clang -Wpessimizing-move

folly: fix clang -Wpessimizing-move

Summary: Make folly `-Wpessimizing-move` clean:

Common errors:
```lang=bash
folly/io/test/NetworkBenchmark.cpp:71:30: error: moving a temporary object prevents copy elision [-Werror,-Wpessimizing-move]
    unique_ptr<IOBuf> next = std::move(head->pop());
                             ^
folly/io/IOBufQueue.cpp:153:28: error: moving a temporary object prevents copy elision [-Werror,-Wpessimizing-move]
      appendToChain(head_, std::move(
                           ^
folly/IPAddressV6.cpp:341:12: error: moving a local object in a return statement prevents copy elision [-Werror,-Wpessimizing-move]
    return std::move(ip);
           ^
folly/IPAddressV6.cpp:341:12: note: remove std::move call here
    return std::move(ip);
           ^~~~~~~~~~  ~
1 error generated.
```

Reviewed By: @fugalh, @meyering

Differential Revision: D2526950

fb-gh-sync-id: 49291a8b49905eb9b2042d004830ff2f599dfbd3