Workaround for MSVC modules bug

[YexuanXiao]

A pattern used in C++/WinRT code runs into an MSVC bug when porting to modules. I reported it here: https://developercommunity.visualstudio.com/t/11070187. This PR is a workaround for the bug, which was discovered in my fork and has been tested and confirmed to work. @DefaultRyan

cppwinrt/strings/base_coroutine_threadpool.h

Lines 160 to 176 in e25fb9a

	void cancel()
	{
	auto canceller = m_canceller.exchange(cancelling_ptr, std::memory_order_acquire);
	struct unique_cancellation_lock
	{
	cancellable_promise* promise;
	~unique_cancellation_lock()
	{
	promise->m_canceller.store(nullptr, std::memory_order_release);
	}
	} lock{ this };
	if ((canceller != nullptr) && (canceller != cancelling_ptr))
	{
	canceller(m_context);
	}
	}