Summary: [Folly] to_shared_ptr.

So you can write this:

    auto sptr = to_shared_ptr(getSomethingUnique<T>());

Instead of this:

    auto sptr = shared_ptr<T>(getSomethingUnique<T>());

Useful when `T` is long, such as `T = foobar::cpp2::FooBarServiceAsyncClient`.

Reviewed By: @meyering

Differential Revision: D2193572