Fix time_point compilation error with Clang

[halflumi]

Attempting to compile the example code in C++/WinRT FAQ with Clang (LLVM 14) and C++/WinRT 2.0.220224.4 results in the following compilation error:

winrt/base.h(8272,77): error: no viable conversion from 'time_point<[...], duration<[...], ratio<[...], __static_lcm<ratio<1, 10000000>::den, ratio<1, 1>::den>::value aka 10000000>>>' to 'const time_point<[...], duration<[...], ratio<[...], 1000000>>>'
            return static_cast<time_t>(std::chrono::system_clock::to_time_t(to_sys(time)));
                                                                            ^~~~~~~~~~~~

This originates from the fact that system_clock uses microseconds in libc++ instead of the 100 nanoseconds in STL of MSVC. A time_point_cast solves the problem. With this change, the example code is able to be compiled in Clang with no other compilation errors.

[kennykerr]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[kennykerr]

Thanks!