assumptions: check C++17 assumption with MSVC From my reading of https://docs.microsoft.com/en-us/cpp/build/reference/zc-cplusplus?view=msvc-160 and https://devblogs.microsoft.com/cppblog/msvc-now-correctly-reports-__cplusplus/ if we set the `/Zc:__cplusplus` switch in additional options, MSVC will report the correct value for `__cplusplus`.
