Fix compilation under Clang

[sylveon]

Fixes #1069

Was manually tested by building and running this snippet:

// silence a clang coroutine warning caused by cppwinrt. we don't actually use coroutines here.
#define _SILENCE_CLANG_COROUTINE_MESSAGE

// std::format is hidden behind /std:c++latest due to ABI concerns right now
// but the headers don't detect Clang's -std=c++2b, so manually have them believe we are using C++23.
#define _HAS_CXX23 1

#include <format>
#include <iostream>

// required for cppwinrt to not fail building with clang
#include <guiddef.h>

#include "winrt/Windows.Foundation.h"

#pragma comment(lib, "onecore.lib")

struct stringable : winrt::implements<stringable, winrt::Windows::Foundation::IStringable>
{
    winrt::hstring ToString()
    {
        return L"a stringable object";
    }
};

int main()
{
    winrt::Windows::Foundation::IStringable obj = winrt::make<stringable>();
    std::wcout << std::format(L"This is {}", obj);
}

With clang -Icppwinrt/_build/x64/Debug -std=c++2b test.cpp

[kennykerr]

/azp run

[azure-pipelines]

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[kennykerr]

Looks good to me. @dunhor ?