Add tearoff support to C++/WinRT

[kennykerr]

This is an advanced feature that most developers won't ever need to deal with. Occasionally there is a need to implement an interface as a COM tearoff and this update adds support for that in C++/WinRT. This also allows C++/WinRT implementations to integrate support for the fast abi forwarding implementation that Scott's been working on. An implementation can simply define the query_interface_tearoff function and provide a suitable implementation. Something like this:

struct Sample : implements<Sample, /* interfaces directly implemented */>
{
    HRESULT query_interface_tearoff(GUID const& id, void** result) const noexcept
    {
        if (is_guid_of</* some tearoff interface */>(id))
        {
            return make_tearoff(this, result);
        }

        *result = nullptr;
        return E_NOINTERFACE;
    }

    // rest of implementation...
};

[Scottj1s]

typo (and below)

[Scottj1s]

could factor a base_tearoff for m_object mgmt, but if these are the only intended test cases, don't bother

[kennykerr]

Yes, just examples really.

[Scottj1s]