Switch to /await:strict when compiling with /stdcpp17

[jlaanstra]

Since 16.10 /await:strict is available: https://docs.microsoft.com/en-us/cpp/build/reference/await-enable-coroutine-support?view=msvc-160

This makes it easier to share C++/WinRT code between code compiled with /stdcpp17 and /stdcpp20 as it makes the coroutine abis compatible.